GRASSLAND BIRD RESPONSE TO

DISKING/INTERSEEDING OF LEGUMES IN

CONSERVATION RESERVE PROGRAM LANDS IN

NORTHEAST NEBRASKA

By

LUCAS PAUL NEGUS

Bachelor of Science

University of Nebraska at Kearney

Kearney, Nebraska

2002

Submitted to the Faculty of the Graduate College of the

Oklahoma State University in partial fulfillment of

the requirements for the Degree of

MASTER OF SCIENCE May, 2006

ii

GRASSLAND BIRD RESPONSE TO

DISKING/INTERSEEDING OF LEGUMES IN

CONSERVATION RESERVE PROGRAM LANDS IN

NORTHEAST NEBRASKA

Thesis Approved:

Craig Davis

Timothy O’Connell

Tim McCoy

A. Gordon Emslie Dean of the Graduate College

iii

ACKNOWLEDGEMENTS

I would like to thank the Nebraska Game and Parks Commission for funding this

project through a State Wildlife Grant. I would specifically like to thank Scott Wessel for

preparing the grant, as well as for providing guidance, advice, and all levels of support

for the project. I also thank Pheasants Forever for providing field equipment and funding

for the project. The Lower Elkhorn Natural Resources District also provided funding for

field technicians, for which I am very thankful. I am also thankful to the Oklahoma State

University Zoology Department for administering the funds for the project.

This project could not have been completed without the assistance and guidance

of many individuals. I sincerely thank my major advisor, Dr. Craig A. Davis, for

guidance, advice, and support through all aspects of this project. I truly appreciate the

assistance and advice my graduate committee members, Dr. Timothy O’Connell and Dr.

Tim McCoy, provided during the study. This project could not have been completed in

entirety without the help of my field technicians, Jamie Bachmann, Cassidy Goc, Jordan

Johnson, and Adam Schole. I am very thankful for their enthusiasm and tireless work

ethic, enduring early, wet mornings and hot afternoons with no complaints. I would also

like to thank the graduate students of the Zoology department for providing help and

support when needed, and especially for the friendship and many good memories. I

would particularly like to thank Sabrina Rust, who lovingly stuck by my side and

supported me in everything I did the last several years. I would like to thank my family

for all the support and visits, as well as my dog, Tyson, for putting up with city life for

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the last couple of years. Finally, I would like to thank my parents for not only letting me

stay at home once again during the summers, but also for instilling the love of the

outdoors in me at an early age and supporting me through everything I have done.

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TABLE OF CONTENTS

Chapter Page ACKNOWLEDGEMENTS ...................................................................................... iii LIST OF TABLES .................................................................................................... vii LIST OF FIGURES ................................................................................................... ix

I. GRASSLAND BIRD RESPONSE TO DISKING/INTERSEEDING LEGUMES IN CONSERVATION RESERVE PROGRAM LANDS IN NORTHEAST NEBRASKA ...........................................................................................................1

Introduction..............................................................................................................1

Grassland Avifauna...............................................................................................1 Habitat Loss and Degredation..........................................................................1 Habitat Fragmentation .....................................................................................3 Conservation Reserve Program.............................................................................5 Justification ..............................................................................................................8 Objectives ..............................................................................................................12 Methods..................................................................................................................12 Study Area ..........................................................................................................12 Disking and Interseeding ....................................................................................13 Bird Abundance Surveys ....................................................................................15 Nest Searches and Monitoring ............................................................................15 Vegetation Sampling...........................................................................................16 Statistical Analysis..............................................................................................17 Results....................................................................................................................21 Grassland Bird Community ................................................................................21 Nesting Success ..................................................................................................23 Vegetation Characteristics ..................................................................................26 Vegetation Influences .........................................................................................27 Discussion ..............................................................................................................28 Conservation Implications and Management.........................................................41 Literature Cited ......................................................................................................46 APPENDICES ............................................................................................................85

vi

Appendix A............................................................................................................85 Appendix B ............................................................................................................87 Appendix C ............................................................................................................89

vii

LIST OF TABLES Table Page 1.1. Overall relative abundance, species richness, and species diversity of breeding

grassland birds in treatment and reference fields in Stanton County, Nebraska, 2004-2005. ....................................................................................................................61

1.2. Relative abundance (birds/transect) of breeding grassland birds that were observed

in >1% of surveys in treatment and reference fields in Stanton County, Nebraska, 2004-2005. ....................................................................................................................62

1.3. Relative abundance (birds/transect) of breeding grassland birds that were observed

in >1% of surveys in 3 portions of Conservation Reserve Program fields managed by disking/interseeding in Stanton County, Nebraska, 2004-2005...............................64

1.4. Number of nesting species, number of nests, and nest density (nests/ha) of grassland

birds in treatment and reference fields in Stanton County, Nebraska, 2004-2005 .......66 1.5. Nest success probabilities for incubation, nestling, and overall nesting period for all

bird species in treatment and reference fields in Stanton County, Nebraska, 2004-2005...............................................................................................................................67

1.6. Nest success probabilities for all bird species, dickcissels, and red-winged

blackbirds in Conservation Reserve Program fields in Stanton County, Nebraska, 2004 and 2005...............................................................................................................68

1.7. Vegetation characteristics of successful and unsuccessful nests of all grassland bird

species in Conservation Reserve Program fields in Stanton County, Nebraska, 2004-2005. ....................................................................................................................69

1.8. Vegetation characteristics of successful and unsuccessful dickcissel nests in

Conservation Reserve Program fields in Stanton County, Nebraska, 2004-2005 ........70 1.9. Vegetation characteristics of successful and unsuccessful red-winged blackbird

nests in Conservation Reserve Program fields in Stanton County, Nebraska, 2004-2005. .............................................................................................................................71

1.10. Vegetation characteristics of successful and unsuccessful grasshopper sparrow nests

in Conservation Reserve Program fields in Stanton County, Nebraska, 2004-2005....72

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1.11. Vegetation characteristics of successful and unsuccessful bobolink nests in

Conservation Reserve Program fields in Stanton County, Nebraska, 2004-2005 ........73 1.12. Vegetation characteristics of treatment and reference fields in Conservation Reserve

Program fields in Stanton County, Nebraska, 2004-2005 ............................................74 1.13. Vegetation characteristics of 3 portions of Conservation Reserve Program fields

managed by disking/interseeding in Stanton County, Nebraska, 2004-2005 ...............75 1.14. Logistic regression models for vegetation variables that best predicted grassland

bird presence in treatment sites in Stanton County, Nebraska, 2004-2005 ..................77 1.15. Logistic regression models for vegetation variables that best predicted grassland

bird presence in reference sites in Stanton County, Nebraska, 2004-2005 ..................78

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LIST OF FIGURES

Figure Page 1.1. Location of study area (represented by black box) in Stanton County, Nebraska........79 1.2. Distance frequency histograms for all species, dickcissels, grasshopper sparrows,

and bobolinks in treatment and reference fields in Stanton County, Nebraska, 2004 and 2005........................................................................................................................81

1.3. Mean (+ S.E.) nest density (nests/ha) for all species combined for 0- (n = 2), 1- (n =

4), and 2-year (n = 2) post treatment and no treatment (n = 11) nest search plots in Conservation Reserve Program fields in Stanton County, Nebraska, 2004 and 2005. Means with different letters are significantly different (P < 0.05) ...............................83

1

CHAPTER I: GRASSLAND BIRD RESPONSE TO DISKING/INTERSEEDING OF LEGUMES IN CONSERVATION RESERVE PROGRAM LANDS IN

NORTHEAST NEBRASKA

INTRODUCTION

Grassland Avifauna

Throughout the Midwest, grassland bird populations are declining faster than any

other group of birds (Samson and Knopf 1994). From 1969 to 1991, grassland bird

populations in Illinois, Minnesota, Wyoming, Nebraska and Missouri declined from 24 to

91% (Samson and Knopf 1994). These declines have been attributed mainly to the loss

of prairie habitat. Samson and Knopf (1994) estimated that as little as 1% of native

prairie habitat remains in the Midwest. Unfortunately, the prairie that remains often

exists in small fragments and receives insufficient management. Additionally, modern

agricultural practices that favor reduced crop diversity and increased field sizes may

contribute to declines in grassland bird numbers (Best et al. 1998).

Habitat loss and degradation—Habitat loss is the primary cause for declines in

grassland bird numbers. There were approximately 162 million ha of prairie in the Great

Plains prior to European settlement, with the Tallgrass Prairie Ecoregion constituting

approximately 60 million of these hectares (Samson and Knopf 1994, Steinauer and

Collins 1996). Today, most of the tallgrass prairie has been plowed and converted to

agricultural lands, with losses as high as 99.9% in some states (Steinauer and Collins

1996). Habitat loss is not the only factor contributing to the decline in grassland birds.

2

Improper or inadequate management of remaining grasslands may also play a role in

grassland bird declines. Fire historically played a major role in the maintenance of

prairies. Periodic fires (every 2-5 years) set by lightning or Native Americans restricted

woody vegetation from encroaching into the prairies and increased plant species diversity

(Steinauer and Collins 1996). Since European settlement, fires have been suppressed

allowing woody vegetation to encroach on and often dominate prairies. The importance

of fire is evident by the response of grassland birds to this disturbance factor. Dechant et

al. (2003) recommended burning every 2-4 years to improve habitat for grasshopper

sparrows (Ammodramus savannarum). Bobolinks (Dolichonyx oryzivorus) also respond

positively to properly timed burns (2-4 yrs) (Herkert 1994). Johnson and Temple (1990)

found lower rates of nest depredation on grasshopper sparrow nests in recently burned

areas in Minnesota.

Many grasslands are included in farming and ranching operations, often to the

detriment of the ecological attributes of those grasslands. Specifically, many agricultural

grasslands and forage crops are mowed or hayed annually, with entire fields being cut in

mid-summer during the peak of nesting season for many grassland birds. Annual

mowing during the breeding season results in high rates of nest failure for dickcissels

(Spiza americana; Frawley and Best 1991). In a study in New York, mowing during the

breeding season accounted for 51% of nest failures for bobolinks (Bollinger et al. 1990).

Annual mowing of entire fields provides habitat for only a select few grassland bird

species, limits changes in vegetation structure, and promotes grass succession (Horn and

Koford 2000, McCoy et al. 2001a). Improper grazing strategies also adversely affect

grassland birds (Zimmerman 1997). The primary impact of grazing is reduction or

3

elimination of above-ground vegetation. Season-long grazing or intensive grazing during

the nesting period reduces vegetation height and density at a critical period, resulting in

reduced avian abundance and productivity (Zimmerman 1997). For example, Swanson

(2003) reported that nesting success of savannah sparrows (Passerculus sandwichensis)

was significantly higher on ungrazed grasslands than on grasslands grazed continuously.

Temple et al. (1999) reported dickcissels were more abundant in ungrazed grasslands

than in continuously or rotationally grazed pastures in southwestern Wisconsin.

Habitat fragmentation—Habitat fragmentation may also be a contributing factor

to declining grassland bird numbers (Herkert 1994, Winter and Faaborg 1999). Johnson

and Igl (2001) defined habitat fragmentation in prairies as the division of large,

contiguous areas of prairie habitat into smaller patches isolated from one another. Three

effects of fragmentation are reduced patch-size, increased edge, and increased isolation

(Johnson and Igl 2001). Several studies have found decreasing patch size to have

negative effects on the presence and nest success of grassland birds (Herkert 1994, Helzer

and Jelinski 1999, Winter and Faaborg 1999, Johnson and Igl 2001). Johnson and Igl

(2001) found 6 of 15 grassland bird species consistently favored larger patches of habitat

over smaller pathces. Herkert et al. (2003) studied nest predation in relation to patch size

for 4 grassland bird species (dickcissel, grasshopper sparrow, Henslow’s sparrow

[Ammodramus henslowii], and eastern meadowlark [Sturnella magna]) and reported

predation rates of these species consistently declined with increasing patch size.

Moreover, predation rates were consistently lowest in prairies larger than 1,000 ha.

Increased edge caused by habitat fragmentation also increases brood parasitism by

brown-headed cowbirds (Molothrus ater). Brown-headed cowbirds, one of the most

4

common edge species in the Great Plains, seem to favor small grassland patches (Johnson

and Igl 2001). Nest productivity is reduced by cowbird parasitism because fewer host

young fledge from parasitized nests than non-parasitized nests (Johnson and Temple

1986). Johnson and Temple (1990) found brood parasitism increased with decreasing

distance to edge. It is speculated that brown-headed cowbirds are more effective near

edges because they use high perches in trees to locate the host’s nests (Johnson and

Temple 1990). Davis and Sealy (2000) observed that cowbird parasitism was highest in

small, irregularly shaped plots with high amounts of edge. Two proposed management

strategies that could reduce cowbird parasitism in grasslands are providing and creating

large tracts of grassland habitat and removing woody edges and woody vegetation from

grasslands (Johnson and Temple 1990, Davis and Sealy 2000, Koford et al. 2000).

The effects of habitat isolation are difficult to quantify. In the field of landscape

ecology, biologists attempt to label isolated habitats as source or sink habitats for animal

populations. A source population is a population in which fecundity is greater than that

required to maintain a stable population. Conversely, a sink population occurs if

fecundity is below the level that is required to maintain a stable population (Pulliam

1988). Although some grasslands have been identified as source or sink habitats for

grassland birds, critical size or specific characteristics of the grassland habitat that

influence source/sink status are not easily identified. Management recommendations

generally encourage large, diverse grasslands to benefit grassland birds, but exact sizes

and habitat conditions required by many species still remain unclear. Johnson and Igl

(2001) suggest that creating grassland habitat near existing grasslands, or establishing 1

large field rather than several small ones would benefit more grassland bird species than

5

creating small, isolated fields. Johnson and Igl (2001) also noted regional differences

may play a role in the development of management strategies (i.e., habitat requirements

in 1 region may not be applicable in another region). Identifying the regional differences

in habitat requirements and developing management strategies is an important component

in the conservation of grassland birds.

Conservation Reserve Program

With the tremendous losses of native prairie throughout the Midwest, surrogate

grasslands such as Conservation Reserve Program (CRP) fields have become increasingly

important to grassland wildlife. The CRP was established as a provision of the Food

Security Act of 1985 (1985 Farm Bill) and has been retained in both the 1996 and 2002

Farm Bills (U.S. Department of Agriculture 2003a). The original goals of CRP were to

reduce erosion and improve water quality of highly erodible cropland, with a co-equal

objective of creating and enhancing wildlife and fish habitat added in 1996 (Johnson and

Schwartz 1993a). CRP pays farmers annual rental payments to retire highly erodible

cropland from production and plant it to grasses, trees, or other perennial cover for 10-15

years (U.S. Department of Agriculture 2003a). CRP differs from previous farm set-aside

programs (e.g., the Payment in Kind Program) because it is a long-term program that

retires cropland for greater than 10 years by planting the cropland to permanent

vegetation providing substantial wildlife benefits (Hays et al. 1989).

In the Midwest, CRP lands have been found to be important to a variety of

grassland wildlife species. Game species such as ring-necked pheasants (Phasianus

colchicus; King and Savidge 1995), northern bobwhite (Colinus virginianus; King and

Savidge 1995), white-tailed deer (Odocoileus virginianus; Luttschwager and Higgins

6

1992) and ducks (Reynolds et al. 1994) all use CRP fields. A study by King and Savidge

(1995) revealed pheasant abundance was higher in areas with a high percentage of CRP

than areas with a low percentage of CRP, probably due to the increased nesting and

brood-rearing habitat. Reynolds et al. (1994) found duck nest success in CRP fields to be

as high or higher than that of cover planted specifically for duck nesting. Nest success

for ducks in CRP fields was 2-9% greater than nest success rates needed to maintain

stable populations.

Non-game grassland songbirds also benefit from CRP (Johnson and Schwartz

1993b, Patterson and Best 1996). Several studies have attributed increases or at least

stable trends in specific grassland bird species to CRP (Igl and Johnson 1995, Herkert

1998, Ryan et al. 1998). Herkert (1998) reported grasshopper sparrow population trends

that were negative in 13 mid-continental states prior to CRP were positive following CRP

enrollment in those states. Igl and Johnson (1995) attributed a rapid increase in Le

Conte’s sparrow (Ammodramus leconteii) in North Dakota to the increased nesting

habitat offered by CRP when favorable wet weather conditions occurred. In an analysis

of North American Breeding Bird Survey results before and after CRP, Reynolds et al.

(1994) concluded that 4 of 8 declining grassland bird species showed positive population

trends, with only 1 species continuing to decline. Not surprisingly, Ryan et al. (1998)

found relative abundance of birds in CRP fields was as much as 10 times higher than in

crop fields. In a similar study, Best et al. (1997) also found the abundance of birds to be

as much as 10.5 times higher in CRP than crop fields. Moreover, CRP fields supported 3

times more nesting species and 13.5 times the total number of nests than row crop fields.

7

Although CRP fields provide habitat for grassland birds, the size and connectivity

of fields influence their suitability for grassland birds. McCoy et al. (1999) determined

whether CRP lands in Missouri were source or sink habitats for grassland bird species.

They found that source-sink status differed among grassland bird species using CRP

fields. Among their results, they found that CRP fields were source habitats for

grasshopper sparrows, field sparrows (Spizella pusilla), eastern meadowlarks, and

American goldfinches (Carduelis tristis) and sink habitats for dickcissels and red-winged

blackbirds (Agelaius phoeniceus). McCoy et al. (1999) concluded that CRP created

“source” habitats for many species, especially where alternative-breeding habitat was in

poor condition or absent from the area. Other studies, however, have documented

declines in grassland bird populations associated with CRP, indicating these lands may

act as a sink habitat (Reynolds et al. 1994). Source/sink dynamics of CRP may be

important to the conservation of grassland birds and need to be investigated more

thoroughly.

Although CRP has benefited numerous wildlife species, the types of plantings

used in CRP fields can have a major effect on grassland bird species. Conservation

Reserve Program fields are planted to a variety of cover types, referred to as conservation

practices (CPs). Two of the most commonly implemented CPs are CP1, a cool-season

grass mix, and CP2, a warm-season grass mix. CP1’s are frequently dominated by

smooth brome (Bromus inermis), orchard grass (Dactylis glomerata), or timothy (Phleum

pratense), while CP2s are frequently dominated by switchgrass (Panicum virgatum). The

vegetation in CP1s is typically short and dense compared to the tall CP2 vegetation.

Recently, natural resource agencies have promoted planting CP2s instead of CP1s to

8

benefit wildlife. However, several studies in the Midwest have found no difference in

total abundance or species richness of grassland birds between CP1 and CP2 plantings

(Johnson and Schwartz 1993b, King and Savidge 1995, Delisle and Savidge 1997,

McCoy et al. 2001b). Although avian richness and abundance may not differ between

CP1s and CP2s, some grassland bird species prefer certain habitat types that may not be

provided separately by each CP. For example, dickcissels prefer CRP fields with tall

grasses and high forb content (Patterson and Best 1996, Hughes et al. 1999), while

grasshopper sparrows prefer habitats of moderate height and density (McCoy et al.

2001a). Ideally, CRP fields should have separate portions planted to CP1 and CP2 to

benefit the full array of grassland bird species.

JUSTIFICATION

Similar to other states, CRP in Nebraska has become increasingly important to

grassland wildlife. The 472,000 ha of CRP in Nebraska is nearly four-times the 123,000

ha of remaining native tallgrass prairie in the state (Steinauer and Collins 1996, U.S.

Department of Agriculture 2003b). This CRP acreage provides additional grassland

habitat and complements the remaining native grasslands for grassland-dependent

species. However, nearly 90% of this CRP acreage in Nebraska is >5 years old (Scott

Wessel, Nebraska Game and Parks Commission, personal communication). As a result,

these older CRP fields have become senescent, providing limited habitat to wildlife.

In eastern Nebraska, typical CP1 plantings in the initial enrollment consisted of a

single cool season grass species, smooth brome, combined with several legume species

(Trifolium spp.) (Clayton Stalling, Nebraska Game and Parks Commission, personal

communication). In the early years of the program, these plantings provided a diverse

9

habitat, consisting of a mixture of grasses, and various forbs and legumes that provided

excellent habitat for many wildlife species. These diverse and heterogeneous grasslands

are desired because they provide multiple niches for a myriad of grassland species.

However, due to succession and the inhibiting nature of smooth brome, these once

productive grassland fields have become a monoculture of smooth brome that has a

limited value to wildlife (Millenbah et al. 1996, McCoy et al. 2001b). Specifically, these

monocultures of smooth brome are characterized by short and dense vegetation with high

amounts of litter accumulation that provide habitat for a limited number of grassland

species.

There have been many studies focused on the effects of mowing, grazing, fire, or

a combination of these management practices on grassland birds in CRP fields

(Zimmerman 1992, Knopf and Samson 1997, Swanson et al. 1999, Horn and Koford

2000, Madden et al. 2000, McCoy et al. 2001a). These management practices can be

effective in enhancing CRP, but are frequently ineffective due to logistical and/or social

constraints. Mowing and haying are often avoided because of the steep terrain and

presence of pocket gophers (Geomys bursarius) in the CRP fields. Pocket gophers make

large mounds of soil, which often damage haying equipment to a point that it may be too

costly to hay or mow (Dale Clark, CRP landowner, personal communication). Grazing is

often avoided because fences must be constructed, water sources provided, and cattle

must be monitored daily. Prescribed burning is rarely used for several reasons. First,

most landowners are not properly trained to conduct prescribed burns, and local fire

marshals are skeptical about providing burn permits to unqualified people. There are also

few burn crews that can provide professional burning services, and those that do are

10

expensive to hire. Finally, fire is still viewed by many landowners as being destructive

and dangerous, even on grasslands.

Because these management practices are rarely used, alternative management

practices have been explored. Plant diversity has been shown to increase through

mechanical means such as disking and interseeding legumes. Shallow disking can be an

effective tool for enhancing CRP fields, acting to set back succession in the fields.

Disking and interseeding is favored by wildlife managers because of the diverse habitat

created and is favored by landowners/farmers because of the availability of equipment

and convenience of the practice. However, disking and interseeding is an ephemeral

practice and provides diverse habitat for only 4-5 years before grass becomes the

dominant vegetation again (Scott Wessel, Nebraska Game and Parks Commission,

personal communication).

In May 2002, the Nebraska Game and Parks Commission initiated a program to

curb declining ring-necked pheasant populations in the state. The program, entitled

“Focus on Pheasants,” placed an emphasis on creating nesting and brood-rearing habitat

in aging CRP fields (Taylor 2002). The best habitats for nesting and brood rearing

include a weedy, diverse grassland. This can most easily be accomplished in CRP fields

by lightly disking the soil to break up the existing mature grass stand which promotes

growth of annual broad-leafed forbs (weeds), increases bare ground, and sets back

succession (Manley et al. 1994). As an added benefit, alfalfa and sweetclover (legumes)

are also interseeded after disking to increase forb abundance and encourage an influx of

insects (Whitmore et al. 1986). Manley et al. (1994) found arthropod biomass to be

significantly higher in disked portions of a field compared to undisked portions. A study

11

conducted by Leathers (2003) evaluated invertebrate abundance on CRP fields in eastern

Nebraska that had been disked and interseeded. Although his results were highly

variable, Leathers (2003) found invertebrate and forb abundance were often higher in

disked and interseeded portions of the fields than untreated CRP fields. The influx in

insects is important because they provide critical food resources for both game bird

chicks and grassland birds (Whitmore et al. 1986, Jackson et al. 1987, Kobal et al. 1998,

McIntyre and Thompson 2003).

Mid-term management, a new rule for CRP sign-up 26 and subsequent sign-ups,

requires management on CRP fields to enhance habitat diversity (U. S. Department of

Agriculture 2003a). Mowing, grazing, burning, and disking/interseeding are all approved

practices for this rule. According to the rule, management must be conducted at least

once on the entire CRP field during the life of the contract, and management can only be

conducted on a maximum of one-third of the field per year. A management regime that

includes disking one-third of a field in years 4, 5, and 6 of a contract could create

desirable field conditions for wildlife by having part of the field in different successional

stages. Millenbah (1993) found avian diversity and density was highest in 1-3 year old

CRP fields in Michigan, which was attributed to the diverse vegetation in the younger

fields.

Mid-term management is also cost-shared under U. S. Department of Agriculture

(USDA) rules. Up to 50% of the incurred cost for the performed management practice

will be reimbursed to the landowner. Cost-share payments combined with the

convenience of disking makes this management attractive to farmers. Resource

professionals in the area expect that mandated management on CRP along with the

12

efforts of conservation agencies and organizations will significantly increase the practice

of disking and interseeding not only in Nebraska, but also throughout the Midwest.

Currently, little knowledge exists on the effects of disking and interseeding on

grassland bird abundance and nest productivity. With CRP fields increasingly providing

key grassland habitat for wildlife throughout the Midwest, disking and interseeding has

the potential to improve the quality of existing CRP fields. While I studied the effects of

disking/interseeding legumes on a fine geographic scale, the response of the grassland

bird community may be extrapolated to a much broader scale to predict the effects of this

management practice throughout the Midwest. This study presents an opportunity to

evaluate grassland bird population response to different levels of succession in grassland

habitats, as well as evaluate the effects of a new management practice on grassland birds.

Moreover, information from this study will be important in guiding future decisions

concerning management of CRP fields and ultimately may influence future USDA

policies for the Conservation Reserve Program.

OBJECTIVES

1) Compare avian richness and abundance in disked/interseeded CRP fields to

unmanaged CRP fields.

2) Compare avian nest productivity in disked/interseeded CRP fields to unmanaged

CRP fields.

3) Evaluate differences in vegetation structure, composition, and cover between

disked/interseeded and unmanaged CRP fields.

METHODS

Study Area

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This study was conducted in Stanton County, Nebraska during the summers of

2004 and 2005 (Figure 1). Stanton County lies in the Loess Uplands and Till Plains

Major Land Resource Area in northeast Nebraska. Precipitation in the area averages 50

to 65 cm per year, with most occurring during the growing season (Natural Resource

Conservation Service 2003). Soils in the area are of the Crofton-Nora and Nora-Crofton

complexes that include silty, loamy, and sandy textural classes (Hammond 1982).

Stanton County is mainly an agricultural county with corn and soybeans being the

predominant crops. Other common crops include oats and alfalfa (Hammond 1982).

Nearly 11,200 ha of CRP occur in Stanton County (U.S. Department of

Agriculture 2003b). This study was conducted on privately-owned CRP within a 51.5

km2 study area. The study area location was selected because of the large percentage of

older CRP fields in the area, the potential to improve habitat quality from management

practices, and landowner cooperation (Taylor 2002). Sixteen CRP fields ranging in size

from 16 to 64 ha were selected for the study. Eight of the fields were manipulated

(treatment fields), and 8 fields were unmanipulated and used as reference fields. A

portion (25-33%) of each manipulated field was disked in 2003 and 2004, resulting in 50-

66% of each field occurring in different successional stages. Conservation Reserve

Program fields in the study area were originally planted to smooth brome, alfalfa, and

yellow sweetclover, but are now dominated by smooth brome. No planned management

(disking, haying, or grazing) has occurred on the reference fields. Selection of study

fields was based on previous management history, field size, proximity to other fields,

and landowner cooperation.

Disking and Interseeding

14

Disking and interseeding of the treatment fields began in September 2002.

Disking was performed by a contractor using a tandem disc designed specifically for sod

breakup. Portions (up to one-third) of each field were selected and flagged prior to

disking. Selection of portions to be disked was based on topography and landowner

preference. Wherever possible, portions that had been hayed in August 2002 were

selected because the reduction of residual cover made disking more effective. Disking

was maintained at a depth of 7.6 to 10.2 cm. Additionally, each portion was disked twice

to effectively break-up the sod. After disking, a no-till drill was used to interseed

legumes. The legume mix, provided by Pheasants Forever, consisted of alfalfa, red

clover, and yellow sweetclover. The legumes were seeded at a rate of 6.75 kg/ha (1.69

kg of red clover, 1.69 kg of yellow sweetclover, and 3.38 kg of alfalfa). According to

USDA guidelines, interseeding was terminated on 15 September 2002 to avoid winter kill

of late germinating plants (Natural Resource Conservation Service 2002). Seeding was

continued after 1 November, when consistent freezes occurred at night. Consistent

freezes ensured a low soil temperature was maintained even during times with warm

daytime temperatures, preventing seed germination and winter-kill. Disking resumed in

April 2003 and continued until 1 May 2003. No disking or interseeding was conducted

between 1 May and 1 August 2003. Disking on another portion of each field resumed in

fall 2003 and was completed in the spring of 2004. All disking depths and seeding rates

were performed in accordance with USDA guidelines (Natural Resource Conservation

Service 2002).

Treatment study fields consisted of CRP fields that had portions disked and

interseeded over the span of 2 years. A typical treatment study field had 25-33% disked

15

in 2003 and 25-33% disked in 2004, with the remainder of the field undisked. The

reference study fields did not have any disking performed on any portion of the field.

Bird Abundance Surveys

I surveyed birds using the belt transect method similar to Best et al. (1997) and

McCoy et al. (2001b). Transects were 200-m long with a fixed 100-m width (Helzer and

Jelinski 1999). Transects were not located <50 m from any field edge or boundary

between disked portions of the field. By maintaining transects >50 m from the edge of

any field (including roads, tree lines, and agricultural fields), I minimized edge effects on

survey results (Helzer 1996). In the treatment fields, I established 3 transects with 1

transect located in each portion of the field (i.e., 2 transects in disked/interseeded portions

and 1 transect in the undisked portion). In the reference fields, I randomly established 2

transects in each field in 2004. In 2005, an additional transect was established in each

reference field to be more consistent with surveys in treatment fields.

I surveyed each transect for avian abundance once during 3 periods (25 May-14

June, 15 June-30 June, and 1 July-15 July) (Best et al. 1997, McCoy et al. 2001b). Avian

abundance was determined by counting birds seen and/or heard while walking along each

transect. All birds encountered within the belt transect were counted. Birds flying over

the transect were noted, but were not included in abundance data for the fields. In 2005, I

recorded the perpendicular distance from the transect to each bird to determine detection

probability for each species encountered. Surveys were conducted between sunrise and 3

hours after sunrise on days when the wind was <16 km/hr with no fog or rain.

Nest Searches and Monitoring

16

In 2 treatment fields, I established a 4-ha nest search plot centered around each

avian survey transect, resulting in 3 nest search plots per field. In 2004, I also established

2, 4-ha nest search plots in reference fields. In 2005, the number of search plots in

reference fields was increased to 6 to be more comparable to treatment fields. Nest

searches were conducted using 4 methods: 1) observing birds engaged in breeding

behavior indicating nest building, incubation, or feeding of nestlings (Martin and Geupel

1993, Giuliano and Daves 2002), 2) chain or rope dragging, 3) systematic walking with

or without a sweeping stick (Winter et al. 2003), and 4) random searches through field

plots (Giuliano and Daves 2002). Plots were searched periodically (>2 times per

sampling period) throughout the summer on a rotational basis to minimize disturbance.

Additional nests were located while conducting avian abundance surveys or sampling

vegetation.

Once a nest was located, it was marked with flagging 5 m to the north and 5 m to

the south of the nest (Giuliano and Daves 2002), and a description of the nest location

was recorded on a nest data sheet. Additionally, Universal Transverse Mercator (UTM)

coordinates for each nest were obtained using a Global Positioning System (GPS) unit.

Nests were checked every 2-5 days until a final outcome was determined (Martin and

Geupel 1993). Outcome was recorded as 1) successful (at least 1 young fledged), 2)

unsuccessful (no young fledged due to depredation, weather, nest parasitism, or unknown

causes), or 3) undetermined.

Vegetation Sampling

Vegetation sampling occurred along the avian abundance survey transects and at

each nest location. I located 4 sampling points (spaced evenly at 40 m, 80 m, 120 m, and

17

160 m) along each transect to record vegetation data. Vegetation data was collected

along survey transects immediately following avian abundance surveys during each

period and at nests within 10 days of nest completion.

At each sampling point on the transect and at each nest, I recorded the following

vegetative characteristics: percent cover, maximum vegetation height, litter depth, and

horizontal visual obstruction. Percent vegetation cover was estimated using a 20 x 50 cm

Daubenmire (1959) frame. Each frame was centered around each sampling point and

percent grass, forb, dead material, and bare ground were estimated (Hughes et al. 1999).

Planted legumes were included with all other forbs in the percent forb cover estimate.

Horizontal visual obstruction was measured using a Robel pole centered on each

sampling point and at each nest (Robel et al. 1970). Horizontal visual obstruction

readings were taken from a height of 1 m and a distance of 4 m from the 4 cardinal

directions. Litter depth and maximum vegetative height within the Daubenmire frame

were measured at each corner of the frame using a meter stick (Hughes et al. 1999).

Additionally, the plant species each nest occurred in and the height of each nest was

recorded.

Statistical Analysis

Prior to conducting statistical analysis, I determined total bird and individual

species relative abundances for treatment and reference fields by averaging count data

from each survey period for each year. Bobolink relative abundance was calculated from

the first and second survey periods because bobolinks were in large groups preparing for

migration during the third survey period. I only included bird species that occurred in

>1% of all surveys for the calculation of individual species abundances. Shannon-

18

Weiner species diversity index was used to calculate overall bird diversity for treatment

and reference fields for each year (Krebs 1999). To examine similarities in grassland

bird communities between treatment and reference fields and among portions of

treatment fields, I used Morisita’s index of similarity (Brower and Zar 1977): IM = 2Σxiyi

/ (λ1 + λ2) N1N2, where xi is the number of individuals in species i for community 1, yi is

the abundance of species i in community 2, λ1 is Simpson’s dominance index for

community 1, λ2 is Simpson’s dominance index for community 2, N1 is the total number

of individuals in community 1, and N2 is the total number of individuals in community 2.

The range of IM is from 0 (no similarity) to 1 (complete similarity). Avian species

richness was also calculated for each treatment and reference field for both years and

included all species observed at any time in study fields during each breeding season.

I used Levene’s test to test for homogeneity of variance for the avian abundance

variables (Zar 1999). Most data sets did not meet assumptions of homogeneity, so I used

a square-root transformation to correct for heteroscedacity (Zar 1999). Because only

total bird abundance met assumptions after transformation, I rank-transformed individual

species data sets that did not meet the assumptions of homogeneity (Conover and Iman

1981). I used a 2-way analysis of variance (ANOVA) to examine differences in total

bird abundance, individual species abundance, diversity, and richness between treatment

and reference fields and years (2004 and 2005) (SAS Institute 2003). To further evaluate

the effects of disking/interseeding on avian species, I also used a 2-way ANOVA to

examine differences in the total bird abundance and individual species abundance of 5

species (dickcissel, red-winged blackbird, bobolink, grasshopper sparrow, and common

yellowthroat) among treatments within each manipulated field (disked-2003, disked-

19

2004, and undisked) and year. I used Duncan’s Multiple Range Test to examine

differences between the 3 portions of treatment fields (SAS Institute 2003).

To assess differences in detectability of individual species between treatment and

reference fields, I constructed frequency histograms with 10-m increments using the

perpendicular distances for each bird observation that was recorded in 2005 (Rotella et al.

1999, McCoy et al. 2001b). Frequency histograms were constructed for all species

combined, as well as for dickcissels, bobolinks, and grasshopper sparrows individually

because they had >20 observations for both treatment and reference fields. Frequencies

were computed by calculating the proportion of observations in each distance increment

for treatment and reference fields. In general, comparisons of these histograms indicated

detectability for all species combined and individual species were similar between

treatment and reference fields (Figure 2).

Nearly all the vegetation data failed to meet assumptions of homogeneity.

Percent dead material was the only vegetation variable that was homogenous. All other

variables (maximum vegetation height; horizontal visual obstruction; litter depth; and

percent forb, grass, and bare ground cover) were square-root transformed (Zar 1999), but

still did not meet assumptions of homogeneity. Therefore, I rank-transformed those

variables (Conover and Iman 1981). I used a 2-way ANOVA to examine differences in

vegetation variables between treatment and reference fields and years. To further

evaluate effects of disking/interseeding on vegetation characteristics, I used a 2-way

ANOVA to examine differences in vegetation variables among different treatments

within each manipulated field and year. I used Duncan’s Multiple Range Test to examine

differences among the 3 portions of treatment fields (SAS Institute 2003).

20

To determine the effect of vegetation characteristics on nest success, I used a 2-

way ANOVA to examine differences in vegetation variables for years and nest fate

(successful and unsuccessful) of all bird species combined, as well as individual

dickcissel, red-winged blackbird, bobolink, and grasshopper sparrow nests. Nest success

for all bird species combined, dickcissels, and red-winged blackbirds was determined

using the Mayfield method (Mayfield 1975, Johnson 1979). Nest success probability

between reference and treatment fields was examined using only all species combined for

2004 and 2005 because <20 nests were found in reference fields in 2004. Dickcissel and

red-winged blackbird nest success probability could not be compared between treatment

and reference fields because <20 nests for each species were found in reference fields

during both years. I used a Chi-square contingency table to examine differences in nest

success, number of nests, and number of species nesting between treatment and reference

fields (Dow 1978, SAS Institute 2003). I used a 2-way ANOVA to test for differences in

nest density between treatment and reference fields and years (SAS Institute 2003).

Additionally, I used a 1-way ANOVA to test for differences in nest density among plots

that were in different post-treatment stages (1- and 2- years post-treatment, current-year

treatment, and no treatment). For all analyses performed, I inferred a significance level

at P < 0.05.

I used logistic regression to develop explanatory models for the occurrence of

individual grassland bird species occurring in >1% of surveys in treatment and reference

fields based on vegetation characteristics (i.e., forb, grass, dead material, and bare ground

cover; horizontal visual obstruction; litter depth; and maximum vegetation height). I

selected logistic regression over linear regression because the individual species

21

abundance data were heavily weighted with zeros and violated assumptions of linear

regression (Zar 1999, Madden et al. 2000). I used the logistic model: P(presence) = 1/ (1

+ exp{ - [bo + b1(x)]}) where P(presence) was the probability that a bird species was

present, bo and b1 were intercept and slope coefficients, and x was the predictor variable

(vegetation variable). I used a backward-elimination routine to create the best model for

each bird species using all vegetation variables. A variable was eliminated from the

model if its observed significance level for the regression coefficient (based on Wald chi-

square) was P > 0.05. I used the Hosmer and Lemeshow (1989) test to assess the

goodness-of-fit of each model.

RESULTS

Grassland Bird Community

Over the 2 years of the study, I observed 28 bird species in treatment fields and 25

species in reference fields (Appendix A). There was high overlap in species assemblages

(IM = 0.72) between treatment and reference fields, yet several species were unique to

treatment and reference fields. Orchard orioles (Icterus spurious), American robins

(Turdus migratorius), cedar waxwings (Bombycilla cedrorum), house sparrows (Passer

domesticus), and killdeer (Charadrius vociferous) were only observed in treatment fields,

while eastern meadowlarks and Baltimore orioles (Icterus galbula) were only observed in

reference fields. Mean abundance of all grassland birds and species diversity were higher

in treatment than reference fields (Table 1). There was a treatment x year interaction for

species richness (F1,28 = 4.47, P = 0.044). In 2004, species richness was higher in

treatment fields, but in 2005, there was no significant difference for species richness

between treatment and reference fields.

22

During the study, 2 species (common yellowthroats [Geothlypis trichas] and red-

winged blackbirds) were significantly more abundant in treatment fields, 2 species

(bobolinks and Henslow’s sparrows) were significantly more abundant in reference

fields, and 1 species (sedge wren [Cistothorus platensis]) had similar relative abundances

in treatment and reference fields (Table 2). Three species (dickcissels [F1,260 = 5.13, P =

0.024], grasshopper sparrows [F1,260 = 8.55, P = 0.004], and western meadowlarks

[Sturnella neglecta] [F1, 260 = 5.29, P = 0.022]), showed significant treatment x year

interactions. Dickcissels were more abundant in treatment fields for each year.

Grasshopper sparrow abundance did not significantly differ between treatment and

reference fields in 2004, but they were more abundant in reference than treatment fields

in 2005. Western meadowlark abundance was significantly higher in treatment fields in

2004, but was not significantly different between treatment and reference fields in 2005.

Dickcissels, grasshopper sparrows, and bobolinks were the most abundant species for

both years in treatment and reference fields, accounting for 78% of the total bird

abundance.

High overlap occurred in grassland bird communities among portions of treatment

fields. Species assemblages between disked-2003 and disked-2004 portions (IM = 0.90)

and between disked-2004 and undisked portions (IM = 0.84) exhibited high overlap. The

least overlap occurred between disked-2003 and undisked portions (IM = 0.66) of

treatment fields. Though Morista’s index of similarity indicated grassland bird

communities in different portions of treatment fields were similar, there were several

differences in bird abundances within treatment fields (Table 3). Bobolinks were

significantly more abundant in the undisked portion than the disked-2003 and disked-

23

2004 portions (P < 0.001). Treatment x year interaction effects were observed for all

other species (all species combined [F2, 138 = 16.76, P < 0.001], dickcissel [F2,138 = 47.98,

P < 0.001], red-winged blackbird [F2,138 = 10.41, P < 0.001], grasshopper sparrow [F2,138

= 5.77, P = 0.004], and common yellowthroat [F2,138 = 10.12, P < 0.001]) (Table 3).

Therefore, subsequent analyses for those species were performed within years. In 2004,

all species combined, dickcissels, red-winged blackbirds, and common yellowthroats

were significantly more abundant in the disked-2003 portion (P < 0.001) than the disked-

2004 or undisked portions of the fields. Grasshopper sparrows were significantly more

abundant in the disked-2004 portion (P = 0.002) of the fields than the disked-2003 and

undisked portions of the fields in 2004. In 2005, all species combined, dickcissels, and

common yellowthroats were significantly more abundant in the disked-2004 portion (P <

0.001) than the disked-2003 or undisked portions. Red-winged blackbirds did not

significantly differ among treatment types (P = 0.522) in 2005. In 2005, grasshopper

sparrows were significantly more abundant in the undisked portion than the disked-2003

portion (P = 0.008), but abundance in the disked-2004 portion did not differ significantly

from the other 2 portions (Table 3). Dickcissels dominated the observations in the

disked-2003 and disked-2004 portions of treatment fields, accounting for 56% and 43%

of bird observations, respectively. In the undisked portions of treatment fields,

grasshopper sparrows were observed most often (34%), followed closely by dickcissels

(27%) and bobolinks (26%).

Nesting Success

I located a total of 247 nests (112 nests in 2004, 135 nests in 2005) during the 2

years of the study. Of those 247 nests, 206 nests of 8 species occurred in treatment fields,

24

and 41 nests of 10 species occurred in reference fields (Table 4). The majority of the

nests located were dickcissel (89) and red-winged blackbird (107) nests. Nest density did

not differ between treatment and reference fields (P = 0.3) or years (P = 0.850) (Table 4).

When further evaluated by years post-treatment instead of by treatment or reference field,

nest density was significantly higher in the 1-year post-treatment plots than the current-

year post-treatment and no treatment plots (P = 0.008). Nest density in the 2-year post-

treatment plots did not differ from any other plots (Figure 3).

Overall nest success probability for all species was similar between treatment and

reference fields (Table 5). Mayfield nest success probability for all bird species was

higher in 2004 than 2005 (Table 6). Dickcissels had nearly twice the nest success

probability in 2004 than 2005, and red-winged blackbirds were nearly 3 times more

successful in 2004 than 2005 (Table 6). Apparent nest success for all birds was 31% in

2004 and 19% in 2005. For dickcissels and red-winged blackbirds, apparent nest success

was 33% and 29% in 2004, and 16% and 9% in 2005, respectively. During both years,

nest failures for all birds were attributed to several causes: depredation (64% - 2004,

75% - 2005), abandonment (8% - 2004, 13% - 2005), weather (7% - 2004, 9% - 2005),

and unknown causes (21% - 2004, 3% - 2005). Dickcissel nest failures were attributed to

depredation (52% - 2004, 73% - 2005), abandonment (7% - 2004, 19% - 2005), weather

(15% - 2004, 3% - 2005), and unknown causes (26% - 2004, 5% - 2005). Red-winged

blackbird nest failures were attributed to depredation (71% - 2004, 75% - 2005),

abandonment (9% - 2004, 12% - 2005), weather (3% - 2004, 2% - 2005), and unknown

causes (17% - 2004, 12% - 2005).

25

I observed many differences in vegetation characteristics between successful and

unsuccessful nests for all species combined (Table 7). Litter depth and percent grass

cover were significantly higher at successful nests. Unsuccessful nests had a

significantly higher percentage of forb cover than successful nests. Litter depth, percent

grass cover, and percent dead material cover were significantly higher at all nests in 2005

than 2004. The percentage of forb cover and bare ground was higher at all nests in 2004

than 2005 (Table 7). A fate x year interaction was observed in nest height (F1,234 = 5.55,

P = 0.019), horizontal visual obstruction (F1,236 = 5.48, P = 0.020), and maximum

vegetation height (F1,236 = 8.19, P = 0.005). Subsequent analyses for these characteristics

were performed within years. In 2004, there was no difference in nest height, horizontal

visual obstruction, or maximum vegetation height between successful and unsuccessful

nests. In 2005, nest height, horizontal visual obstruction, and maximum vegetation

height were significantly higher at unsuccessful nests than successful nests.

There were few differences in vegetation characteristics between successful and

unsuccessful dickcissel nests (Table 8). Fate x year interactions were observed for

maximum vegetation height (F1,78 = 6.26, P = 0.014) and percent dead material (F1,78 =

4.09, P = 0.047). Further analyses were performed within years for these characteristics.

In 2004, neither maximum vegetation height nor percent dead material differed between

successful and unsuccessful dickcissel nests. In 2005, maximum vegetation height was

higher at unsuccessful than successful dickcissel nests. Nest height, horizontal visual

obstruction, percent forb cover, and percent bare ground were significantly higher at

dickcissel nests in 2004 than 2005. Litter depth and percent grass cover were

significantly higher at dickcissel nests in 2005 than 2004. Nest height did not differ

26

between successful and unsuccessful dickcissel nests, but dickcissel nests were higher in

2004 than 2005 (Table 8). Red-winged blackbird nests showed several differences

between successful and unsuccessful nests (Table 9). No fate x year interactions were

observed. Litter depth and percent grass cover were higher at successful than

unsuccessful red-winged blackbird nests (Table 9). Percent forb cover was higher at

unsuccessful than successful red-winged blackbird nests. Horizontal visual obstruction,

maximum vegetation height, percent forb cover, and percent bare ground were higher at

all nests in 2004 than 2005. Litter depth and percent grass cover were higher at red-

winged blackbird nests in 2005 than 2004. Grasshopper sparrow nests exhibited 1

difference between successful and unsuccessful nests; percent bare ground was higher at

successful than unsuccessful grasshopper sparrow nests (Table 10). There were no

significant differences between successful and unsuccessful bobolink nests (Table 11).

Vegetation Characteristics

Many vegetation characteristics differed between treatment and reference fields

(Table 12). Forb cover was significantly higher in treatment fields, while grass cover was

significantly higher in reference fields. Grass cover was higher in all fields (treatment

and reference) in 2005 than 2004. Treatment x year interactions occurred in horizontal

visual obstruction (F1,1040 = 8.65, P < 0.001), litter depth (F1,1040 = 30.17, P < 0.001),

maximum vegetation height (F1,1040 = 61.70, P < 0.001), percent dead material (F1,1040 =

25.20, P < 0.001), and percent bare ground (F1,1040 = 47.13, P < 0.001). Further analyses

of these vegetation characteristics were performed within years. Maximum vegetation

height, horizontal visual obstruction, and percent bare ground were all significantly

higher in treatment fields than reference fields in 2004 and 2005. Litter depth and

27

percent dead material were significantly higher in reference than treatment fields in 2004

and 2005 (Table 12).

There were also many differences in vegetation characteristics within treatment

fields (Table 13). Treatment x year interactions occurred in all analyses (horizontal

visual obstruction [F2,570 = 104.12, P < 0.001]; litter depth [F2,570 = 81.33, P < 0.001];

maximum vegetation height [F2,570 = 170.57, P < 0.001]; percent forb [F2,570 = 128.05, P

< 0.001], grass [F2,570 = 46.53, P < 0.001], dead material [F2,570 = 18.59, P < 0.001], and

bare ground [F2,570 = 56.38, P < 0.001] cover). Thus, further analyses were performed

within years. Horizontal visual obstruction was highest in the disked-2003 portion of the

treatment fields for both years (P < 0.001) (Table 13). Litter depth and percent dead

material were highest in the undisked portion of the treatment fields for 2004 and 2005 (P

< 0.001). In 2004, litter depth was similar between disked-2003 and disked-2004

portions, while in 2005 disked-2003 portions had more litter than disked-2004 portions.

In 2004, maximum vegetation height was highest in the disked-2003 portion, but in 2005,

maximum vegetation height was highest in disked-2003 and disked-2004 portions (P <

0.001). Forb cover was highest in the disked-2003 portion of treatment fields in 2004 (P

< 0.001), but shifted to being highest in the disked-2004 portion in 2005 (P < 0.001). In

2004, grass cover was higher in the undisked portion of the fields than the other 2

portions (P < 0.001). But, in 2005, grass cover was highest in undisked and disked-2003

portions (P < 0.001). Percent bare ground was highest in the disked-2004 portion of the

fields in 2004 and 2005 (P < 0.001) (Table 13).

Vegetation Influences

28

Vegetation had considerable influence on the presence of individual grassland

bird species in CRP fields. In treatment fields, 7 of the 8 most common grassland bird

species had significant explanatory models (Table 14). Presence of all the species, except

red-winged blackbirds, was best explained by more than 1 variable. Red-winged

blackbird occurrence was explained by increasing maximum vegetation height.

Dickcissel occurrence was best explained by 2 variables, increasing forb cover and visual

obstruction. Common yellowthroat occurrence was explained by increasing forb cover

and vegetation height, while sedge wrens were associated with increasing forb cover and

litter depth. Bobolinks were associated with increasing dead material and decreasing

bare ground. Western meadowlarks were associated with decreasing forb and grass

cover. Grasshopper sparrows were associated with increasing forb cover and litter depth,

and decreasing vegetation height (Table 14). Henslow’s sparrows did not have a

significant model in treatment fields.

In reference fields, 3 of the 8 most common grassland bird species had significant

explanatory models (Table 15). Common yellowthroats were associated with increasing

vegetation height, and red-winged blackbirds were associated with increasing litter depth.

Bobolink occurrence was explained by 2 variables, increasing grass and dead material

cover. Dickcissels, grasshopper sparrows, Henslow’s sparrows, sedge wrens, and

western meadowlarks did not have significant explanatory models in reference fields.

DISCUSSION

Overall, CRP grasslands in northeastern Nebraska provided habitat for a wide

variety of grassland birds. Of the 30 bird species observed in my study fields, 8 were

grassland bird species that commonly occurred in the fields and used the fields for

29

nesting, foraging, and raising young. In an analysis of North American Breeding Bird

Surveys (BBS) from 1966-1993, Herkert (1995) reported that over 75% of all grassland

bird species, including the 8 most commonly observed species in my study, have shown

significant declines throughout the Midwest. Recent BBS data (1966-2004) indicate that

grasshopper sparrows, western meadowlarks, dickcissels, and common yellowthroats

continue to decline (Sauer et al. 2005). However, these more recent data also show an

increasing trend for bobolinks and sedge wrens and a stable or a slightly decreasing trend

for red-winged blackbirds. Henslow’s sparrows, which were only observed in my

reference fields, may be one of the fastest declining grassland birds (Herkert 1997),

especially in the east and northeast portions of its range (Pruitt 1996). Recently,

however, local Henslow’s sparrow populations in the Midwest have increased (Sauer et

al. 2005), possibly due to habitat created by CRP (Herkert et al. 2002). These recent BBS

results, along with my findings, suggest that CRP grasslands will continue to be

important for these grassland bird species into the future and may be critical in

maintaining or increasing current populations.

Although CRP fields appear to provide critical habitat for grassland birds, the lack

of appropriate management often reduces their value to individual species. Disking and

interseeding creates a diverse mosaic of vegetation that positively benefits many

grassland bird species. Avian species diversity, richness (in 2004), and mean abundance

of all species combined were higher in the treatment than reference fields, indicating the

vegetation structure resulting from treatment of fields accommodated more species than

the reference fields. Millenbah et al. (1996) found similar results in newly planted CRP

fields, presumably because of the vegetation changes from the disturbance of planting

30

them. In my study, disked/interseeded portions of treatment fields had a diverse

vegetation composition and structure, consisting of various heights of forbs, weeds, and

grasses. Additionally, treatment fields consisted of 3 portions that were in different

vegetation successional stages. This diverse vegetation composition and structure within

disked/interseeded portions and variety of successional stages throughout treatment fields

is likely responsible for the high avian diversity, richness, and abundance.

Dickcissels were the most common grassland birds in my study area and were

most abundant in treatment fields. Dickcissels prefer grasslands that have dense, tall

cover that provide many elevated song perches (Hughes et al. 1999, Temple 2002,

Dechant et al. 2003). Additionally, areas with high proportions of forbs, especially

legumes, provide nesting cover, nest support (Appendix B), and an increased abundance

of invertebrate foods (Frawley and Best 1991, Patterson and Best 1996, Temple 2002).

Treatment fields provided the dense, tall legumes important to dickcissels for nesting and

foraging, along with live and dead forbs (sunflowers [Asteraceae], ragweed [Asteraceae],

hemp [Cannabaceae], and thistles [Asteraceae]) that were frequently used for perches

(Lucas Negus, personal observation). More specifically, the disked/interseeded portions

of the treatment fields, particularly the 1-year post-treatment portions, provided

vegetation structure that was nearly ideal for dickcissels. This preference for the

vegetation structure in the 1-year post-treatment portions within treatment fields is shown

by the consistent high abundance of dickcissels in the 1-year post-treatment portions of

treatment fields over the 2 years of the study. Dickcissels were significantly more

abundant in the 1-year post-treatment portion of treatment fields during both years of the

study. Red-winged blackbirds and common yellowthroats were also more abundant in

31

treatment fields. Like dickcissels, both species showed preferences for the 1-year post-

treatment portion of the treatment fields during both years of the study. Red-winged

blackbirds, which generally breed in a wide range of wetland and upland habitats

(Yasukawa and Searcy 1995), were observed almost exclusively in treatment fields.

They commonly placed their nests high (> 50 cm) in the legumes, and used live and dead

forbs for perching. Common yellowthroats also used the tall vegetation for perching, but

were more commonly observed hidden in the lower, dense legumes. This preference for

the low, dense vegetation by common yellowthroats is consistent with that reported by

Stewart (1953) for common yellowthroats in Michigan.

Grasshopper sparrows showed no preference between treatment and reference

fields in 2004, but preferred reference fields in 2005. Grasshopper sparrows generally

prefer grasslands of moderate height and density with patchy bare ground (Vickery 1996,

McCoy et al. 2001a). With the exception of bare ground, reference fields and undisked

portions of the treatment fields offered this suitable grassland habitat of moderate height

and density. However, within treatment fields in 2004, grasshopper sparrows displayed a

preference for areas with bare ground and were significantly more abundant in the

portions of treatment field that had been disked in the spring 2004 than any other field

portions. In the disked portions, grasshopper sparrows were frequently observed perched

on dirt clumps, where vegetation was sparse and short or intermediate in height

throughout the season and bare ground was abundant (Lucas Negus, personal

observation). In 2005, disked/interseeded portions of treatment fields had undergone at

least 1 full growing season, resulting in the majority (50 – 66%) of each field having tall,

32

dense vegetation that was less suitable for grasshopper sparrows compared to reference

fields.

Western meadowlarks were more abundant in treatment fields in 2004, but

showed no difference between treatment and reference fields in 2005. The preference of

treatment fields in 2004 was somewhat unexpected and contradictory to the findings of

Wiens and Rotenberry (1981), who reported western meadowlarks preferred areas with

high amounts of grass and litter cover. Reference fields were characterized more by high

amounts of grass and litter than treatment fields. Perhaps the treatment fields provided

adequate areas of grass and litter cover for nesting and were more attractive for foraging

than reference fields. In 2005, western meadowlarks showed no preference between

treatment and reference fields, possibly indicating that the vegetation changes in

treatment fields between 2004 and 2005 resulted in less desirable vegetation in 2005.

Bobolinks and Henslow’s sparrows were more abundant in reference fields than

treatment fields during both years of the study. Both species prefer habitats with high

percentages of grass and dead material, and high amounts of litter (Bollinger and Gavin

1992, Herkert and Glass 1999). This habitat was primarily found in reference fields and

the portion of treatment fields that was undisked. Henslow’s sparrows were most often

observed perched and nesting on hilltops where vegetation was not as high or dense as it

was in valleys. This is contrary to the findings of Zimmerman (1988) who found

Henslow’s sparrow males in Kansas set up territories in areas with taller vegetation than

surrounding areas that were unused. This may indicate Henslow’s sparrows use a wider

range of habitats than originally thought. Bollinger and Gavin (1992) found bobolinks in

New York preferred old hayfields (>8 years old), and fields >30 ha supported twice the

33

bobolink density of fields <10 ha. The reference fields in my study were all >10 ha and

had not been disturbed for >10 years; it is not surprising bobolink abundance was higher

in these fields than the treatment fields that had smaller undisked portions. Although

results from my study primarily support Bollinger and Gavins’ (1992) findings, bobolinks

were also observed nesting in small (<10 ha) undisked portions of treatment fields, as

well as disked/interseeded portions of the fields. This indicates that although treatment

fields were not used as much as reference fields were used by bobolinks, they still

provided adequate nesting habitat for a reduced number of bobolinks.

Not surprisingly, vegetation characteristics significantly affected presence of

many of the grassland bird species in CRP fields. Nearly every species (7 of 8 species)

had significant explanatory models in treatment fields, while only 3 species (red-winged

blackbirds, bobolinks, and common yellowthroats) had significant models in reference

fields. Reference fields likely had few explanatory models because vegetation was very

uniform throughout the entire fields, and species presence could not be attributed to

distinct vegetation characteristics. Dickcissels and common yellowthroats were

associated with similar vegetation variables in treatment fields. Dickcissels were

positively associated with vegetation height and forb cover, while common yellowthroats

were positively associated with visual obstruction and forb cover in treatment fields.

Both species were most commonly observed in disked/interseeded portions of the

treatment fields that had undergone at least 1 full growing season. Although these 2

species were associated with similar vegetation variables and were commonly observed

in the same portions of the treatment fields, they were likely able to coexist without

competition in these portions because the vertical vegetation structure present provided a

34

separate foraging space for each species. Cody (1968) concluded that vegetation height

was important because avian species foraging strategies are stratified by the vertical

vegetation structure. Dickcissels used the upper levels of the vertical structure in CRP

fields, while common yellowthroats used the lower levels. Maximum vegetation height

has been reported as being an important factor for dickcissels (Zimmerman 1966) and

was likely a key variable in my study because dickcissels were commonly observed

perching and nesting high in the vegetation. In addition to the height of vegetation, the

amount of forb cover present was also important for dickcissels. Delisle and Savidge

(1997) concluded dickcissel abundance was positively correlated to forb occurrence.

Forbs were used by dickcissels for both perching and nesting during my study. Visual

obstruction was likely a key variable for common yellowthroats because they prefer the

low, dense vegetation (Stewart 1953). A high visual obstruction measurement would

indicate the vegetation is both tall and dense, providing larger amounts of the preferred

dense vegetation. Common yellowthroats were most likely associated with increasing

forbs because they provided this tall, dense vegetation.

Red-winged blackbirds were positively associated with maximum vegetation

height in treatment fields and litter depth in reference fields. It was not unexpected that

red-winged blackbirds would be associated with increasing vegetation height in treatment

fields since they frequently selected the tallest vegetation for nest placement (Lucas

Negus, personal observation). The tall vegetation also provided prominent perch sites for

males to display and to alert others when potential predators are near (Yasukawa and

Searcy 1995). In reference fields, red-winged blackbirds were quite often observed in

transects parallel to or crossing valleys or water-ways in fields. These valleys and water-

35

ways generally consisted of dense, lush stands of grass compared to other areas in the

fields, presumably due to the higher amounts of moisture in these areas. Because of the

density of the vegetation in these areas, litter accumulation was considerably higher in

these areas than other areas of the field, explaining the positive association with litter

depth in reference fields.

Grasshopper sparrow presence in treatment fields was associated with increasing

forb cover and litter depth, and decreasing vegetation height. This model fits well with

observations reported by others. While habitat preferences of grasshopper sparrows vary

by region, they generally prefer somewhat disturbed areas with short or intermediate

vegetation height (Vickery 1996, Dechant et al. 2003). Whitmore (1981) noted

grasshopper sparrows preferred sparse, patchy habitat and nested in vegetation clumps

with high amounts of litter, and Schneider (1998) found increasing litter to be one of the

strongest predictors of grasshopper sparrow presence in grasslands.

Bobolinks were associated with increasing dead material cover and decreasing

bare ground in treatment fields and increasing grass and dead material cover in reference

fields. These predictive models seem to fit very well with the abundance surveys and

observations from this study, as well as other studies. In New York, bobolink density

was higher in areas with low legume cover and high amounts of litter and grass cover

(Bollinger and Gavin 1992). Similar to Henslow’s sparrows, bobolinks used undisked

CRP more often than disked/interseeded CRP. With no disturbance, the undisked CRP

had high amounts of litter, dead material, and grass cover.

Sedge wrens were associated with 2 vegetation variables in treatment fields,

increasing forb cover and litter depth. Sedge wrens prefer areas with tall, dense

36

vegetation (Johnson and Schwartz 1993b, Herkert et al. 2001). Delisle and Savidge

(1997) also found sedge wrens were positively associated with litter depth in Nebraska.

In treatment fields, the disked/interseeded portions had high amounts of forbs that likely

provided the tall, dense vegetation that was preferred by sedge wrens rather than the

undisked portions.

Western meadowlarks in treatment fields were negatively associated with forb

and grass cover, which is somewhat puzzling. Western meadowlarks occur in a diversity

of grassland habitats, but are generally thought to prefer areas of short, less dense grasses

(Lanyon 1994, McCoy 1996). One explanation may be that the forbs were too tall and

dense in disked portions of treatment fields. In a study on Montana grasslands, Dieni and

Jones (2003) reported that western meadowlarks avoided nesting in patches with high

percentages of forb cover. Another explanation may be that landscape characteristics of

treatment fields such as patch size, core area, and edge area, which were not included in

the model, were responsible for the presence of western meadowlarks rather than the

measured vegetation characteristics that were used to predict presence. Perhaps the best

explanation for the model comes from Wiens and Rotenberry (1981), who reported that

western meadowlarks did not correlate well with any habitat characteristics.

Vegetation characteristics also likely influenced nest densities in my study fields.

The nest density in treatment plots was nearly 3 times the nest density in reference plots.

Additionally, the nest plots in the undisked portion of the treatments fields had very low

densities (0.77 nests/ha), while the plots in disked/interseeded portions had moderate to

very high nest densities (3.00 – 6.25 nests/ha). A further evaluation of nest densities by

years post-treatment may better represent the differences in nest density. The density of

37

nests peaked in the 1-year post-treatment plots and decreased in all other stages of

treatment (no treatment, current-year, and 2-year post-treatment) (Figure 3). This trend is

due to the high number of red-winged blackbird and dickcissel nests that were observed

nearly exclusively in the disked/interseeded portions of treatment fields. As mentioned

above, the vegetation characteristics of disked/interseeded portions of treatment fields

were nearly ideal for dickcissels, and red-winged blackbirds were attracted to the same

characteristics. Dickcissels and red-winged blackbirds are both polygynous and as many

as 15 red-winged blackbird females have been reported nesting in 1 male territory,

resulting in high nest densities (Zimmerman 1966, Yasukawa and Searcy 1995).

Additionally, it appeared both species re-nested in the same area soon after a nest failure

rather than leaving the disked/interseeded portions of fields (Lucas Negus, personal

observation). This is somewhat contrary to that reported by Zimmerman (1982), who

reported only 17% of dickcissel females re-nested in the same territory they originally

nested in. This use of the same field portions for re-nesting may have also contributed to

the high nest density in the disked/interseeded portions of treatment fields.

Although differences in nest density between treatment and reference fields were

detected, there were no differences in nest success probability between treatment and

reference fields. The vegetation characteristics of treatment fields positively influenced

both abundance and nest density of many grassland bird species, but appear to not

enhance the nest success. The lack of difference in nest success suggests that vegetation

characteristics that were attractive for nesting did not offer more protection from

predators than reference field vegetation.

38

As mentioned, there was no difference in nest success probability between

treatment and reference fields, but nest success probability of all species combined and

red-winged blackbirds in 2005 was only half of that in 2004. Possible factors that may

have caused the decline in nest success include climate/weather differences, increased

search effort during the 2005 field season, increased nest densities, increased predator

numbers, and improved predator search images. Climate/weather differences were not

likely responsible; the weather conditions in 2004 and 2005 were similar and nest failure

due to weather was similar between the 2 years. In 2005, I hired 2 additional field

technicians that increased our nest searching efforts. Although we had more people in

the fields in 2005, the same methods for finding and checking nests were used as were

used the previous summer, so there should not have been an increase in disturbance

affecting nest success probability in 2005.

Although previous research is inconclusive, nest density may be correlated to

depredation rates. Robertson (1972) reported predation rates decreased with increasing

nest density, while Fretwell (1977) reported increased predation rates with increased

density. Zimmerman (1984) found no correlation between dickcissel nest density and

predation rates. Regardless, nest densities did not differ between 2004 and 2005, thus

nest densities likely had little influence on the nest success probability difference between

the 2 years.

Because the percentage of nests that were depredated increased from 64% in 2004

to 75% in 2005, it is possible that predators may have increased in abundance or

developed better search images in the 2005 field season. Although nest success

probability was similar between treatment and reference fields, more nests were

39

depredated in treatment fields simply because there were many more nests (primarily red-

winged blackbird and dickcissel nests) in these fields. Because treatment fields were first

disked/interseeded in 2004, it was the first time a large number of red-winged blackbirds

and dickcissels nested in these CRP fields. Therefore, it is possible that predator

populations may have been low or had not developed the search image to find these

particular nests. After the influx of nests in 2004, predator populations may have

increased or predators may have developed a search image for nests by the 2005 field

season.

Others have suggested yearly predation differences are due to fluctuations in

predator populations (Davis 2003). In a study on Saskatchewan grasslands, Davis (2003)

partially attributed abnormally low nest success in 1997 to an influx of prairie voles

(Microtus ochrogaster) during that year. While the prairie voles were not always the nest

predator, they likely also attracted other predators that foraged opportunistically on

grassland bird nests. Additionally, predators select prey from a large prey community

that includes grassland birds. Annual changes in grassland bird nest predation may be

reflective of changes in the overall prey community. Schmidt (1999) predicted that

increasing abundance of alternative foods (prey) would decrease grassland bird nest

depredation. A study on artificial nest success in CRP fields by Vander Lee et al. (1999)

supported this prediction, reporting increased nest success in plots in which supplemental

prey was supplied. While I did not evaluate changes in the prey community during this

study, it is likely that the prey community, like the bird and vegetation communities,

changed over the years of the study due to the disking/interseeding.

40

Overall, 71% of the nests were depredated over the 2 years of this study. This is

similar to findings of other studies in the Midwest (Patterson and Best 1996, Winter

1999). In my study, few of the nests were disturbed or destroyed, suggesting the

predators involved were most likely snakes or small mammals (Thompson et al. 1999,

Pietz and Granfors 2000). A depredated field sparrow nest was observed with 1 dead,

decapitated young, suggesting a mouse likely depredated the nest (Pietz and Granfors

2000). Thirteen-lined ground squirrels (Spermophilus tridecemlineatus) and Franklin’s

ground squirrels (Spermophilus franklinii), known nest predators, were also frequently

observed in the fields.

Other nests were unsuccessful due to abandonment, weather, or unknown causes.

Surprisingly, no nests were unsuccessful due to cowbird parasitism. Only a few nests

were parasitized over the 2 years of the study, but all of these nests were depredated

before the young fledged. This is surprising, however, because power lines, shelter belts,

and farm buildings, all reported as perch fields for cowbirds (Johnson and Temple 1990,

Schaffer et al. 2003), are abundant in the study area. Most likely, very few nests were

parasitized simply because there were very few cowbirds in the area. My survey results

indicated there was a very low abundance of cowbirds throughout the study area.

Vegetation characteristics also may have affected nest success of individual

species. For dickcissels, the only vegetation difference between successful and

unsuccessful nests occurred in 2005. Contrary to what was expected, maximum

vegetation height around unsuccessful nests was higher than that around successful nests.

It is expected that higher vegetation around nests would provide better nest concealment,

increasing the probability of nest success. However, this pattern may indicate predators

41

that use olfactory senses or audio cues rather than visual cues, such as snakes, were

responsible for depredation. Zimmerman (1984) concluded in his study on dickcissels in

Kansas that snakes were most likely a major predator on nests and nest density or

concealment of nests were not important factors influencing the rate of predation.

Instead, snake density was likely the most important factor influencing depredation. In

my study, most depredated nests were not disturbed or destroyed indicating snakes could

have been the primary nest predator (Thompson et al. 1999).

Nest success of the 2 most abundant ground-nesting species (grasshopper

sparrows and bobolinks) did not appear to be significantly affected by vegetation.

Successful grasshopper sparrow nests had more bare ground surrounding them than

unsuccessful nests. Whitmore (1981) reported grasshopper sparrow territories had higher

amounts of bare ground than areas not used for territories. Grasshopper sparrows

generally nested in clumps of vegetation, usually surrounded by bare ground. This also

indicates grasshopper sparrows may be more successful in areas that have a diversity of

grasses, forbs, and bare ground, rather than an evenly distributed monoculture of

vegetation. Bobolink nests exhibited no vegetation differences between successful and

unsuccessful nests. Bobolinks primarily nested in undisked CRP, where there were few

forbs and vegetation was uniform throughout the field. Contrary to other studies,

bobolinks in my study concealed their nests in litter surrounded by evenly-distributed

grasses and not in clumps of forbs or grasses (Martin and Gavin 1995). This is likely the

reason there were no vegetation differences between successful and unsuccessful

bobolink nests.

CONSERVATION IMPLICATIONS AND RECOMMENDATIONS

42

As indicated by my results and those of many others, CRP fields provide critical

habitat for grassland breeding birds (Johnson and Schwartz 1993b, Igl and Johnson 1995,

Best et al. 1997, Delisle and Savidge 1997). However, as CRP fields age without

management, their benefits to grassland birds may decrease (Millenbah et al. 1996).

Aging CRP fields can be enhanced using several management methods including haying,

burning, grazing, and disking/interseeding legumes (McCoy et al. 2001b). In my study,

disking/interseeding legumes was promoted primarily because of the diverse vegetation

created in CRP fields dominated by smooth brome compared to other practices.

Although the costs of management vary by region and practice, disking/interseeding is

generally more expensive than the other practices, with fuel costs contributing greatly.

However, if entire CRP fields are hayed or grazed, a 25% reduction in CRP payments is

imposed (U. S. Department of Agriculture 2003a). Depending on land rental rates, this

reduction in CRP payments may make haying or grazing more costly than

disking/interseeding. Burning is the most cost effective of the practices, but is rarely

used in the region because of lack of burn crews, landowner experience, and social

factors (fear of burning). Additionally, burning smooth brome does not create the

vegetation diversity that can be created using disking/interseeding (Scott Wessel,

Nebraska Game and Parks Commission, personal communication).

According to my results, disking/interseeding legumes benefited many grassland

bird species in several ways. Disked/interseeded fields accommodated higher

abundances, more species, and higher diversities of grassland birds than the reference

fields. The manner in which the fields were disked/interseeded seemed to create a range

of vegetation characteristics, or successional stages, that were desirable to a wide array of

43

grassland bird species. Disking/interseeding produced the weedy, sparse vegetation that

is similar to that of newly seeded CRP (Millenbah et al. 1996). In the first growing

season following disking, typical vegetation characteristics included little to no litter,

high amounts of bare ground, sparse and short stature legumes (alfalfa and red clover)

early in the season, moderately dense and medium height planted legumes later in the

season, with patches of unplanted forbs (sunflowers, ragweed, hemp, and thistles)

occurring throughout the field (Lucas Negus, personal observation). Grasses present in

the first growing season were typically annuals such as foxtails (Setaria spp.).

Vegetation in the 1-year post-treatment portions was predominantly tall, dense planted

legumes. Yellow sweetclover provided tall vegetation stature, whereas the red clover and

alfalfa filled in the lower levels of the vertical structure. There were few other forbs and

grasses present after 1 full growing season. However, many unplanted forb stalks

remained from the previous season and were frequently used for perching. In the 2-years

post treatment portions, the vegetation consisted mainly of tall smooth brome, with sparse

alfalfa and red clover interspersed in the lower levels of the vegetation. There was little

bare ground in the third year after disking. Reference fields were typically dominated by

short to medium height smooth brome, few or no forbs, moderate amounts of standing

dead material, high amounts of litter, and no bare ground. Vegetation characteristics in

reference fields did not change during the 2 years of the study.

It is likely factors other than vegetation, such as patch characteristics and

invertebrate communities, had impacts on the grassland bird community in the CRP

fields. Core area, field or patch size, edge area, and distance to edge have been reported

to affect grassland bird communities throughout the Midwest (Herkert 1994, Helzer and

44

Jelinski 1999, Winter and Faaborg 1999, Sporrong 2004). While I did not examine patch

characteristics such as field or patch size and edge area, it is possible that these factors

influenced the grassland bird communities in my study. Moreover, it is also possible that

disked/interseeded portions were too small to accommodate some area sensitive species

such as grasshopper sparrows, bobolinks, and western meadowlarks (Helzer and Jelinski

1999, Winter and Faaborg 1999). Additionally, the increased amount of edge created in

treatment fields by disking/interseeding portions of the fields may have reduced nest

success due to increased predation. Winter et al. (2000) suggested that dickcissels and

other habitat generalists are affected by decreased nest success near edges due to

increased depredation (primarily by mesocarnivores) near these edges. However, nearly

all “edge” studies focused on “hard” edges such as tree rows or roads, whereas the edges

in my project were “soft” edges, representing changes in herbaceous vegetation structure.

Further research on the effects of “soft” edges on grassland birds is needed. I attempted

as best as I could to control for field size by selecting similar sized reference and

treatment fields, as well as selecting treatment fields in which portions were

disked/interseeded in similar sizes and shapes, but ultimately could not select fields that

were identical in size and shape. Although not likely, these small differences in size and

shape may have influenced bird communities in my treatment and reference fields.

Invertebrates are important food resources for breeding grassland birds, as well as

many other bird species (Hull et al. 1996). Several studies have reported that grassland

birds opportunistically forage on invertebrates from the orders Coleoptera, Lepidoptera,

Orthoptera, and Aranae (Kaspari and Joern 1993, Kobal et al. 1998, McIntyre and

Thompson 2003). An examination of the invertebrate community in my study fields in

45

2004 indicated treatment fields provided higher abundances and biomasses of these

orders than reference fields (Appendix C). Grassland birds are likely not only responding

to the vegetation differences in the treatment fields, but also to the invertebrate

community that is influenced by the vegetation changes.

To maximize vegetation and grassland bird diversity in CRP fields, I recommend

using disking/interseeding legumes as a management tool. Moreover, I recommend

establishing an annual rotation of this management practice to maximize benefits. An

annual rotation would create a mosaic of vegetation characteristics, which would provide

habitat for several species simultaneously (Herkert 1996, Madden et al. 2000, Winter et

al. 2005). Ideally, fields could be divided into fourths, with one-fourth of the field being

disked/interseeded annually. To accommodate the suite of grassland birds, I also

recommend maintaining one-fourth of the field out of the annual rotation to provide the

mature grassland habitat that is preferred by several species (e.g., Henslow’s sparrows

and bobolinks) (Herkert 1996). Additionally, fields that are difficult to disk/interseed

because of rough terrain, poor accessibility, or other reasons should not be

disked/interseeded, thus providing important habitat to those species that may be

displaced by disking/interseeding.

This study was relatively short in duration, and the benefits of

disking/interseeding beyond 2 years after the initial disking/interseeding are unknown.

Petersen and Best (1999) concluded that 2-3 years of post-disturbance data for

perturbation experiments may be insufficient for grassland birds. Vegetation

characteristics indicate that in the third year after disking/interseeding, vegetation

becomes more similar to that in reference fields. It can be predicted that the grassland

46

bird community will likely reflect these vegetation changes as the field matures.

However, exactly how long it takes for grassland bird communities in treatment fields to

become similar to those of reference fields is not known. Researching the

disked/interseeded fields for several years (> 5) following management would yield more

confident conclusions.

Future grassland bird studies in CRP should not only focus on vegetation

characteristics, but should also focus on the landscape characteristics and invertebrate

communities as mentioned previously. Additionally, studies should be long-term (> 3

years) (Petersen and Best 1999) and cover a wide geographic range (Winter et al. 2005).

Grassland habitats vary highly between years and regions, as do grassland bird

populations. Thus it is important that studies extend over wide geographic regions and

years to accurately measure grassland bird communities (Igl and Johnson 1999). These

types of studies are important to better understand grassland bird populations and trends

in the future, especially as required management of CRP becomes more common and

widespread. Future studies on management practices, specifically disking/interseeding,

should also examine source-sink population implications. Although disked/interseeded

portions had higher abundances and nest densities than undisked portions, they may be

acting as sink habitats rather than source habitats. Finally, future studies of CRP

vegetation management should focus on techniques that will increase the longevity of the

vegetation created by disking/interseeding. Disking/interseeding legumes may be more

effective when used in conjunction with other management techniques such as herbicide

application, burning, haying, or grazing.

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61

Table 1. Overall relative abundancea, species richnessb, and species diversityc of

breeding grassland birds in treatment and reference fields in Stanton County, Nebraska,

2004-2005.

a Mean abundance of all bird species from 3 sampling periods during each year. b Number of avian species observed in each field. c Shannon-Weiner species diversity index (Krebs 1999). d P = P-value for treatment (treatment vs. reference) and year (2004 vs. 2005) effects

from a 2-way analysis of variance. Interaction effects (treatment x year) were observed

for species richness (F1,28 = 4.47, P = 0.044). Means for species richness are reported

separately for each year.

Treatment (n = 8)

Reference (n = 8)

Pd

Year Mean S.E. Mean S.E. Treatment Year Overall abundance (no./transect)

2004-2005

4.49 0.25 2.93 0.21 <0.001 0.150

2004 10.13 0.48 6.00 0.53 <0.001 Species richness

2005 10.38 0.63 8.75 0.70 0.106 Species diversity

2004-2005

1.41 0.06 1.23 0.05 0.034 0.229

62

Table 2. Relative abundancea (birds/transect) of breeding grassland birds that were

observed in >1% of surveys in treatment and reference fields in Stanton County,

Nebraska, 2004-2005.

Treatment (n = 8)

Reference (n = 8)

Pc

Speciesb Year Mean S.E. Mean S.E. Treatment Year DICK 2004

2005

1.81

2.29

0.26

0.22

0.60

0.61

0.11

0.12

0.008

<0.001

GRSP 2004 2005

1.29

0.47

0.15

0.09

1.04

0.96

0.15

0.10

0.261

<0.001

BOBO 2004- 2005

0.57

0.09 1.44 0.16 <0.001 0.917

RWBL 2004-2005

0.52 0.10 0.03 0.01 <0.001 0.920

COYE 2004-2005

0.22 0.04 0.03 0.01 <0.001 0.170

SEWR 2004-2005

0.07 0.03 0.09 0.04 0.484 0.186

WEME 2004 2005

0.26

0.04

0.07

0.02

0.04

0.04

0.03

0.02

0.015

1.000

HESP 2004-2005

0.01 0.01 0.08 0.03 <0.006 0.349

a Mean abundance from 3 sampling periods each year. b DICK = dickcissel, RWBL = red-winged blackbird, BOBO = bobolink, GRSP =

grasshopper sparrow, COYE = common yellowthroat, SEWR = sedge wren, WEME =

western meadowlark, and HESP = Henslow’s sparrow. c P = P-value for treatment (treatment vs. reference) and year (2004 vs. 2005) effects

from a 2-way analysis of variance. Interaction (treatment x year) effects occurred for

63

dickcissels (F1,260 = 5.13, P = 0.024), grasshopper sparrows (F1,260 = 8.55, P = 0.004), and

western meadowlarks (F1,260 = 5.29, P = 0.022).

64

Table 3. Relative abundancea (birds/transect) of breeding grassland birds occurring in

>1% of surveys in 3 portions of Conservation Reserve Program fields managed by

disking/interseeding in Stanton County, Nebraska, 2004-2005.

Disked-2003 (n = 8)

Disked-2004 (n = 8)

Undisked (n = 8)

Speciesc Year Mean S.E. Mean S.E. Mean S.E. All Species

2004 2005

7.96ab

3.88b

0.69

0.45

3.33b

6.08a

0.21

0.52

3.00b

2.71b

0.43

0.43 DICK

2004 2005

4.25a

2.42b

0.40

0.31

0.33b

3.75a

0.16

0.31

0.83b

0.71c

0.24

0.19 RWBL 2004

2005

1.75a

0.38a

0.41

0.15

0.04b

0.59a

0.04

0.20

0.13b

0.25a

0.09

0.11 BOBO 2004-

2005 0.41b 0.14 0.22b 0.14 1.09a 0.16

GRSP 2004 2005

0.75b

0.17b

0.22

0.08

2.00a

0.42ab

0.23

0.15

1.13b

0.83a

0.26

0.20 COYE 2004

2005

0.42a

0.25b

0.12

0.09

0.00b

0.58a

0.00

0.15

0.04b

0.00c

0.04

0.00 a Mean abundance from 3 sampling periods each year. b Means in rows with different letters were different (P < 0.05). c DICK = dickcissel, RWBL = red-winged blackbird, BOBO = bobolink, GRSP =

grasshopper sparrow, and COYE = common yellowthroat. d P = P-value for treatment (disked 2003 vs. disked 2004 vs. undisked) effects from a 2-

way analysis of variance. Treatment x year interaction effects occurred in all species

combined (F2, 138 = 16.76, P < 0.001), dickcissels (F2,138 = 47.98, P < 0.001), red-winged

blackbirds (F2,138 = 10.41, P < 0.001), grasshopper sparrows (F2,138 = 5.77, P = 0.004),

65

and common yellowthroats (F2,138 = 10.12, P < 0.001). Means are reported separately for

each year.

66

Table 4. Number of nesting species, number of nests, and nest density (nests/ha) of

grassland birds in treatment and reference fields in Stanton County, Nebraska, 2004-

2005.

Treatment (n = 6)

Reference (n = 6)

Year Mean S.E. Mean S.E. Number of species

2004 2005

5

8

--

--

4 9

--

-- Number of nests

2004 2005

100

106

--

--

12

29

--

-- Nest density (nests/ha)

2004- 2005

2.79 1.10 0.91 0.28

67

Table 5. Nest success probabilitiesa for incubation, nestling, and overall nesting period

for all bird species in treatment and reference fields in Stanton County, Nebraska, 2004-

2005.

a Determined using Mayfield (1975) method. b P = P-value for chi-square analysis of overall nest success probability (Dow 1978).

Treatment (n = 159)

Reference (n = 30)

Pb

Incubation(%)

Nestling(%)

Overall(%)

Incubation(%)

Nestling(%)

Overall(%)

All bird species

47.64 43.29 20.62 52.55 38.57 20.27

0.971

68

Table 6. Nest success probabilitiesa for all bird species, dickcissels, and red-winged

blackbirds in Conservation Reserve Program fields in Stanton County, Nebraska, 2004-

2005.

a Determined using Mayfield (1975) method. b DICK = dickcissel and RWBL = red-winged blackbird. c P = P-value for chi-square analysis of overall nest success probability (Dow 1978).

2004 (n = 93)

2005 (n = 96)

Pc

Speciesb Incubation(%)

Nestling(%)

Overall(%)

Incubation(%)

Nestling(%)

Overall(%)

All bird species

57.77 46.43 26.82 41.32 37.98 15.28 0.053

DICK 64.19 40.36 25.91 31.77 44.64 14.18 0.232

RWBL 52.81 51.12 27.00 41.94 23.52 9.87 0.045

69

Table 7. Vegetation characteristics of successful and unsuccessful nests of all grassland

bird species in Conservation Reserve Program fields in Stanton County, Nebraska, 2004-

2005.

Successful (n = 60)

Unsuccessful (n = 180)

Pa

Vegetation characteristic

Year Mean S.E. Mean S.E. Fate Year

Nest height (cm) 2004 2005

36.06

16.00

4.68

3.96

37.71

34.22

2.76

2.14

0.749

<0.001

Visual obstruction (dm)

2004 2005

6.55

3.84

0.47

0.34

6.79

5.69

0.29

0.20

0.649

<0.001

Litter depth (cm) 2004-2005

1.95 0.24 1.64 0.13 0.036 <0.001

Maximum vegetation height (cm)

2004 2005

100.93

67.12

5.46

3.99

100.44

87.79

2.79

2.20

0.690

<0.001

Forb (%) 2004-2005

34.00 4.47 47.71 2.71 0.001 <0.001

Grass (%) 2004-2005

49.50 3.66 41.96 2.23 0.015 <0.001

Dead material (%) 2004-2005

12.83 2.52 9.21 1.15 0.057 0.010

Bare ground (%) 2004-2005

4.17 1.14 1.57 0.39 0.602 <0.001

a P = P-value for fate (successful vs. unsuccessful) and year (2004 vs. 2005) effects from

2-way analysis of variance. Interaction (fate x year) effects occurred in maximum

vegetation height (F1,236 = 8.19, P = 0.005), nest height (F1,234 = 5.55, P = 0.019), and

visual obstruction (F1,236 = 5.48, P = 0.020). Means are reported separately for each year

for these variables.

70

Table 8. Vegetation characteristics of successful and unsuccessful dickcissel nests in

Conservation Reserve Program fields in Stanton County, Nebraska, 2004-2005.

Successful (n = 20)

Unsuccessful (n = 62)

Pa

Vegetation characteristic

Year Mean S.E. Mean S.E. Fate Year

Nest height (cm) 2004-2005

31.25 3.64 33.31 1.73 0.171 0.002

Visual obstruction (dm)

2004-2005

6.80 0.50 6.46 0.24 0.560 <0.001

Litter depth (cm) 2004-2005

1.31 0.37 1.40 0.24 0.438 <0.001

Maximum vegetation height (cm)

2004 2005

112.62

72.86

6.27

7.23

102.27

89.80

4.12

3.03

0.148

0.043

Forb (%) 2004-2005

60.50 7.27 56.77 4.29 0.751 0.005

Grass (%) 2004-2005

31.00 5.75 37.10 3.77 0.928 0.006

Dead material (%) 2004 2005

0.00

12.86

0.00

7.14

2.31

6.11

1.28

1.34

0.213

0.123

Bare ground (%) 2004-2005

3.00 2.52 1.45 0.64 0.755 0.032

a P = P-value for fate (successful vs. unsuccessful) and year (2004 vs. 2005) effects from

2-way analysis of variance. Interaction (fate x year) effects occurred in maximum

vegetation height (F1,78 = 6.26, P = 0.014) and dead material cover (F1,78 = 4.09, P =

0.047). Means are reported separately for each year for these variables.

71

Table 9. Vegetation characteristics of successful and unsuccessful red-winged blackbird

nests in Conservation Reserve Program fields in Stanton County, Nebraska, 2004-2005.

Successful (n = 18)

Unsuccessful (n = 86)

Pa

Vegetation characteristic

Year Mean S.E. Mean S.E. Fate Year

Nest height (cm) 2004-2005

55.17 4.14 49.37 1.95 0.483 0.257

Visual obstruction (dm)

2004-2005

6.94 0.40 7.04 0.20 0.226 0.003

Litter depth (cm) 2004-2005

1.39 0.40 1.19 0.15 0.031 <0.001

Maximum vegetation height (cm)

2004-2005

111.61 4.21 103.88 1.70 0.903 <0.001

Forb (%) 2004-2005

41.11 6.36 56.98 3.39 0.013 0.041

Grass (%) 2004-2005

53.89 6.67 37.67 3.12 0.007 0.030

Dead material (%) 2004-2005

1.11 0.76 4.42 0.80 0.281 0.078

Bare ground (%) 2004-2005

4.44 2.46 1.41 0.44 0.912 0.005

a P= P-value for fate (successful vs. unsuccessful) and year (2004 vs. 2005) effects from

2-way analysis of variance. No interaction (fate x year) effects occurred.

72

Table 10. Vegetation characteristics of successful and unsuccessful grasshopper sparrow

nests in Conservation Reserve Program fields in Stanton County, Nebraska, 2004-2005.

Successful (n = 7)

Unsuccessful (n = 12)

Pa

Vegetation characteristic

Mean S.E. Mean S.E. Fate

Nest height (cm) 0.00 0.00 0.00 0.00 --

Visual obstruction (dm)

1.86 0.18 2.45 0.23 0.093

Litter depth (cm) 2.04 0.73 3.08 0.36 0.167

Maximum vegetation height (cm)

47.54 3.56 51.17 3.57 0.513

Forb (%) 0.00 0.00 0.83 0.83 0.461

Grass (%) 61.43 7.04 60.83 4.68 0.943

Dead material (%) 30.00 9.51 38.33 5.20 0.412

Bare ground (%) 11.43 5.95 0.83 0.83 0.055

a P = P-value for nest fate effects from 1-way analysis of variance.

73

Table 11. Vegetation characteristics of successful and unsuccessful bobolink nests in

Conservation Reserve Program fields in Stanton County, Nebraska, 2004-2005.

Successful (n = 7)

Unsuccessful (n = 8)

Pa

Vegetation characteristic

Mean S.E. Mean S.E. Fate

Nest height (cm) 0.00 0.00 0.00 0.00 -- Visual obstruction (dm)

3.50 0.22 3.16 0.13 0.190

Litter depth (cm) 3.57 0.30 3.94 0.33 0.430

Maximum vegetation height (cm)

56.29 2.99 61.41 3.56 0.298

Forb (%) 0.00 0.00 0.00 0.00 --

Grass (%) 70.00 5.77 65.00 5.67 0.549 Dead material (%) 32.86 7.14 36.25 7.30 0.747

Bare ground (%) 0.00 0.00 2.50 1.64 0.179

a P = P-value for nest fate effects from 1-way analysis of variance.

74

Table 12. Vegetation characteristics of treatment and reference fields in Conservation

Reserve Program fields in Stanton County, Nebraska, 2004-2005.

Treatement (n = 8)

Reference (n = 8)

Pa

Vegetation characteristic

Year Mean S.E. Mean S.E. Treatment Year

Visual obstruction (dm)

2004 2005

3.04

4.11

0.15

0.08

2.67

3.21

0.07

0.05

0.032

<0.001

Litter depth (cm) 2004 2005

1.51

1.98

0.11

0.10

3.34

2.82

0.09

0.09

<0.001

<0.001

Maximum vegetation height (cm)

2004 2005

57.60

73.78

1.91

1.16

57.46

54.30

1.03

0.56

0.966

<0.001

Forb (%) 2004-2005

23.80 1.29 1.43 0.32 <0.001 0.407

Grass (%) 2004-2005

41.81 1.16 63.93 0.85 <0.001 <0.001

Dead material (%) 2004 2005

27.36

23.06

1.46

1.10

48.13

30.51

1.69

1.03

<0.001

<0.001

Bare ground (%) 2004 2005

25.00

3.96

1.70

0.51

2.03

0.83

0.37

0.27

<0.001

<0.001

a P = P-value for treatment (treatment vs. reference) and year (2004 vs. 2005) effects

from 2-way analysis of variance. Interaction (treatment x year) effects occurred in

horizontal visual obstruction (F1,1040 = 8.65, P < 0.001), litter depth (F1,1040 = 30.17, P <

0.001), maximum vegetation height (F1,1040 = 61.70, P < 0.001), percent dead material

(F1,1040 = 25.20, P < 0.001), and percent bare ground (F1,1040 = 47.13, P < 0.001).

75

Table 13. Vegetation characteristics of 3 portions of Conservation Reserve Program

fields managed by disking/interseeding in Stanton County, Nebraska, 2004-2005.

Disked-2003 (n = 8)

Disked-2004 (n = 8)

Undisked (n = 8)

Vegetation characteristics

Year Mean S.E. Mean S.E. Mean S.E.

Visual obstruction (dm)

2004 2005

5.44aa

4.70a

0.26

0.13

0.50c

4.31b

0.08

0.17

3.17b

3.32c

0.10

0.09

Litter depth (cm)

2004 2005

0.34b

2.33b

0.05

0.19

0.39b

0.71c

0.06

0.10

3.80a

2.89a

0.12

0.15

Maximum vegetation height (cm)

2004 2005

90.31a

81.79a

2.38

1.98

23.96c

80.72a

1.49

1.85

58.54b

58.83b

1.42

1.17

Forb (%) 2004 2005

58.44a

17.29b

2.96

1.98

16.15b

49.27a

2.36

3.20

0.42c

1.25c

0.25

0.60 Grass (%) 2004

2005

21.88b

59.90a

2.11

2.39

14.90c

29.27b

2.01

2.09

62.50a

62.40a

1.40

1.64 Dead material (%)

2004 2005

11.04c

19.48b

1.47

1.61

22.92b

14.48c

2.24

1.77

48.13a

35.21a

2.17

1.68 Bare ground (%)

2004 2005

19.48b

3.33b

1.70

0.78

53.54a

7.40a

2.88

1.16

1.98c

1.15c

0.72

0.44 a Means in rows with different letters were different (P < 0.05). b P = P-value for treatment (disked 2003 vs. disked 2004 vs. undisked) effects from a 2-

way analysis of variance. Treatment x year interaction effects occurred in all analyses

(horizontal visual obstruction [F2,570 = 104.12, P < 0.001]; litter depth [F2,570 = 81.33, P <

0.001]; maximum vegetation height [F2,570 = 170.57, P < 0.001]; percent forb [F2,570 =

76

128.05, P < 0.001], grass [F2,570 = 46.53, P < 0.001], dead material [F2,570 = 18.59, P <

0.001], and bare ground [F2,570 = 56.38, P < 0.001] cover).

77

Table 14. Logistic regression models for vegetation variables that best predicted

grassland bird presence in treatment fields in Stanton County, Nebraska, 2004-2005.

Variables were selected from a set of vegetation variables using a backward-elimination

routine.

Speciesa Fitted logistic modelbc Pd

DICK -3.59 + 0.07 (forb) + 1.13 (vo)

0.313

RWBL -3.94 + 0.04 (max ht)

0.837

GRSP 2.78 + 0.04 (forb) + 0.41 (litter) – 0.07 (max ht)

0.409

BOBO -1.22 + 0.04 (dead) - 0.08 (bare)

0.816

COYE -6.13 + 0.02 (forb) + 0.05 (max ht)

0.621

WEME 0.98 – 0.03 (forb) – 0.07 (grass)

0.587

SEWR -6.65 + 0.05 (forb) + 1.00 (litter)

0.944

HESP No Significant Model

--

a DICK = dickcissel, RWBL = red-winged blackbird, BOBO = bobolink, GRSP =

grasshopper sparrow, COYE = common yellowthroat, SEWR = sedge wren, WEME =

western meadowlark, HESP = Henslow’s sparrow. b P(presence) = 1/ (1 + exp{ - [bo + b1(x)]}) where P(presence) was the probability that a

bird species was present, bo and b1 were intercept and slope coefficients, and x was the

predictor variable (vegetation variable), and Absence(x) = 1 – Presence(x). c forb = forb cover, grass = grass cover, dead = dead material cover, bare = bare ground,

vo = horizontal visual obstruction, max ht = maximum vegetation height, and litter =

litter depth. d P = P-value for Hosmer and Lemeshow goodness-of-fit test of overall model.

78

Table 15. Logistic regression models for vegetation variables that best predicted

grassland bird presence in reference fields in Stanton County, Nebraska, 2004-2005.

Variables were selected from a set of vegetation variables using a backward-elimination

routine.

Speciesa Fitted logistic modelbc Pd

DICK No Significant Model

--

RWBL -7.59 + 1.06 (litter)

0.914

GRSP No Significant Model

--

BOBO -7.76 + 0.09 (grass) + 0.08 (dead)

0.305

COYE -21.71 + 0.27 (max ht)

0.999

WEME No Significant Model

--

SEWR No Significant Model

--

HESP No Significant Model

--

a DICK = dickcissel, RWBL = red-winged blackbird, BOBO = bobolink, GRSP =

grasshopper sparrow, COYE = common yellowthroat, SEWR = sedge wren, WEME =

western meadowlark, HESP = Henslow’s sparrow. b P(presence) = 1/ (1 + exp{ - [bo + b1(x)]}) where P(presence) was the probability that a

bird species was present, bo and b1 were intercept and slope coefficients, and x was the

predictor variable (vegetation variable), and Absence(x) = 1 – Presence(x). c forb = forb cover, grass = grass cover, dead = dead material cover, bare = bare ground,

vo = horizontal visual obstruction, max ht = maximum vegetation height, and litter =

litter depth. d P = P-value for Hosmer and Lemeshow goodness-of-fit test of overall model.

79

Figure 1. Location of study area (represented by the black box) in Stanton County,

Nebraska.

80

NEBRASKA

81

Figure 2. Distance frequency histograms for all species combined, dickcissels,

grasshopper sparrows, and bobolinks in treatment and reference fields in Stanton County,

Nebraska, 2005.

82

Freq

uenc

yofo

bserv

ation

s

All species

0

0.1

0.2

0.3

0 10 20 30 40 50

Distance interval (m)

Bobolink

0

0.1

0.2

0.3

0.4ReferenceTreatment

Grasshopper sparrow

0

0.1

0.2

0.3

Dickcissel

0

0.1

0.2

0.3

0.4

0.5

83

Figure 3. Mean (+ S.E.) nest density (nests/ha) for all species combined for 0- (n = 2), 1-

(n = 4), and 2-year (n = 2) post treatment and no treatment (n = 11) nest search plots in

Conservation Reserve Program fields in Stanton County, Nebraska, 2004 and 2005.

Means with different letters are significantly different (P < 0.05).

84

0123456789

10

0 1 2 No treatmentYears post treatment

Nest

s/ha

A

AB

BB

85

APPENDIX A

Bird species observed in Conservation Reserve Program fields in Stanton County,

Nebraska, 2004-2005.

Species

Treatment Reference

Grasshopper sparrow (Ammodramus savannarum)

X X

Dickcissel (Spiza Americana) X X

Bobolink (Dolichonyx oryzivorus) X X

Common yellowthroat (Geothlypis trichas) X X

Red-winged blackbird (Agelaius phoeniceus) X X

Western meadowlark (Sturnella neglecta) X X

Eastern kingbird (Tyrannus tyrannus) X X

Sedge wren (Cistothorus platensis) X X

Henslow’s sparrow (Ammodramus henslowii) X X

Barn swallow (Hirundo rustica) X X

Tree swallow (Tachycineta bicolor) X X

Northern rough-winged swallow (Stelgidopteryx serripennis)

X X

Mourning dove (Zenaida macroura) X X

Common grackle (Quiscalus quiscula) X X

Mallard (Anas platyrhychos) X XAmerican goldfinch (Carduelis tristis) X X

Field sparrow (Spizella pusilla) X X

Killdeer (Charadrius vociferous) X

House sparrow (Passer domesticus) X

86

Species

Treatment Reference

Blue-winged teal (Anus discors) X X

Brown-headed cowbird (Molothrus ater) X X

Ring-necked pheasant (Phasianus colchicus) X X

European starling (Sturnus vulgaris) X X

Northern flicker (Colaptes auratus) X X

Northern harrier (Circus cyaneus) X X

Orchard oriole (Icterus spurious) X

American robin (Turdus migratorius) X

Cedar waxwing (Bombycilla cedrorum) X

Eastern meadowlark (Sturnella magna) X

Baltimore oriole (Icterus galbula) X

87

APPENDIX B

Percentage of each plant species used as a primary vegetationa support for nest placement

by the 4 most common nesting grassland bird species in Conservation Reserve Program

fields in Stanton County, Nebraska, 2004-2005.

Plant Species Dickcissel (n = 89)

Red-winged blackbird (n = 106)

Grasshopper sparrow (n = 19)

Bobolink (n = 15)

Yellow sweetclover (Melilotus officinalis)

36 52

Alfalfa (Medicago sativa)

17 9

Red clover (Trifolium pratense)

11 4

Mustard spp. (Brassica spp.)

2 9

Smooth brome (Bromus inermis)

21 14 84 100

Thistle spp. (Carduus spp.) or (Cirsium spp.)

9 4

Curly doc (Rumex crispus)

5

Kentucky bluegrass (Poa pratensis)

5

Orchard grass (Dactylis glomerata)

1 11

Nettle spp. (Urtica spp.)

1

Sunflower spp. (Helianthus spp.)

1

Milkweed spp. (Asclepias spp.)

1

Wild plum (Prunus americana)

1

88

Plant Species Dickcissel

Red-winged blackbird

Grasshopper sparrow

Bobolink

Chinese elm (Ulmus parvifolia)

1

a In addition to the primary plant species used for nest placement, 36% of red-winged

blackbirds and 39% of dickcissels used a secondary plant species to help support the nest.

Secondary plant species were most commonly alfalfa (25%), smooth brome (23%),

yellow sweetclover (22%), and red clover (18%).

89

APPENDIX C

Relative abundance and biomass of invertebrates in disked/interseeded and undisked

portions of Conservation Reserve Program fields in Stanton County, Nebraska, 2004.

Disked-2003 (n = 8)

Disked-2004 (n = 8)

Undisked (n = 24)

Pa

Mean S.E. Mean S.E. Mean S.E. Period TreatmentAbundance 176.00ab 63.38 192.88a 50.94 79.58b 21.07 0.11 0.05 Biomass 1.29ab 0.38 1.53a 0.59 0.64b 0.09 0.02 0.03

a P = P-value for period (1 vs. 2) and treatment (disked-2003 vs. disked-2004 vs.

undisked) effects from 2-way analysis of variance. No interaction (period x treatment)

effects occurred. b Means in rows with different letters were different (P < 0.05).

VITA

Lucas Paul Negus

Candidate for the Degree of

Master of Science

Thesis: GRASSLAND BIRD RESPONSE TO DISKING/INTERSEEDING OF LEGUMES IN CONSERVATION RESERVE PROGRAM LANDS IN NORTHEAST NEBRASKA

Major Field: Wildlife and Fisheries Ecology Biographical:

Personal Data: Born in Tilden, Nebraska, 1980 Education: Elkhorn Valley High School, 1998; Bachelors of Science from

University of Nebraska-Kearney in May, 2002; Masters of Science candidate at Oklahoma State University in May, 2006.

Experience: Fish and Wildlife Biologist, Nebraska Game and Parks

Commission, 2002-2003; Conservation Technician, Nebraska Game and Parks Commission, 1999-2002; Work Study, Nebraska Game and Parks Commission, 1998-2002; Family farm employee.

Professional Memberships: Member of National chapter, Nebraska, and

Oklahoma state chapters of The Wildlife Society, Pheasants Forever, Oklahoma State University Zoology Graduate Student Society, Oklahoma State University student chapters of The Wildlife Society and American Fisheries Society.

Name: Lucas Paul Negus Date of Degree: May, 2006 Institution: Oklahoma State University Location: Stillwater, Oklahoma Title of Study: GRASSLAND BIRD RESPONSE TO DISKING/INTERSEEDING OF

LEGUMES IN CONSERVATION RESERVE PROGRAM LANDS IN NORTHEAST NEBRASKA

Pages in Study: 89 Candidate for the Degree of Master of Science

Major Field: Wildlife and Fisheries Ecology Scope and Method of Study: Throughout the Midwest, grassland birds have been

declining faster than any other group of birds, with the main cause for these declines being the extensive loss of native prairies. During the last 25 years, surrogate grasslands, such as Conservation Reserve Program (CRP) lands, have become increasingly important as an alternative habitat for grassland birds. However, many CRP grasslands that once provided excellent habitat are now dominated by monoculture stands of grass that have reduced wildlife benefits. In summer 2000, the Nebraska Game and Parks Commission and Pheasants Forever partnered with the United States Department of Agriculture’s Farm Service Agency and Natural Resource Conservation Service to initiate a program that promotes disking and interseeding legumes in CRP to improve nesting and brood-rearing habitat for ring-necked pheasants (Phasianus colchicus). Disking and interseeding will also likely affect grassland birds, yet there has been no research on their response to this practice. The objectives of this study were to determine grassland bird abundance and nest-productivity in disked and interseeded CRP, evaluate vegetative responses to disking and interseeding, and provide management recommendations that may influence future farm bill policy regarding CRP grasslands.

Findings and Conclusions: Findings indicate experimental fields had higher overall avian

species richness, abundance, and diversity than reference fields. Dickcissels, red-winged blackbirds, and common yellowthroats were more abundant in treatment fields while bobolinks and Henslow’s sparrows were more abundant in reference fields. Nest densities in experimental fields were nearly 3 times greater than nest densities in reference fields. To increase grassland bird diversity and abundance, I recommend establishing an annual rotation of disking/interseeding to create a mosaic of vegetation types, while leaving portions of CRP fields undisked to accommodate species that don’t respond positively to management.

ADVISER’S APPROVAL: Craig A. Davis