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Have Cowbirds Caused Forest Songbirds to Decline?Author(s): Margaret Clark Brittingham and Stanley A. TempleSource: BioScience, Vol. 33, No. 1 (Jan., 1983), pp. 31-35Published by: Oxford University Presson behalf of the American Institute of Biological SciencesStable URL: http://www.jstor.org/stable/1309241.

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2/6

H a v e Cowbirds Caused

F o r e s t

Songbirds

o

Decline

Margaret

Clark

Brittingham

and

Stanley

A.

Temple

Brown-headed

cowbird

populations

and

their

rate of brood

parasitism

on forest

songbirds

in eastern

North

America

have increased

since

1900. Brood

parasitism

of

forest

songbirds

is

highest

near

open

habitat.

High

brood

parasitism

rates within

isolated

fragments

of forest habitat reduce

reproductive

success

of certain forest

songbirds

and

may

be

responsible

for their recent declines.

(Accepted for publica-

tion

10

July

1982)

Brood

parasites

lay

their

eggs

in the

nests

of host

species

that hatch

and rear

the

parasite's

young.

Obligate

brood

par-

asites never rear

their

own

young

and

are, therefore,

totally

dependent

on

host

species.

The

brown-headed cowbird

(Molothrus

ater)

is the

only

obligate

brood

parasite

in North America.

Brown-headed

cowbirds

and other

brood

parasites

substantially

reduce the

reproductive

success

of their

hosts;

few-

er than normal or often none of the

host's

young fledge

from a

parasitized

nest

(Payne

1977,

Mayfield

1977a).

In

response

to

this

threat,

many

host

spe-

cies

that coevolved

with

brood

parasites

can reduce the

success of

parasites by

deserting

a

parasitized

nest,

building

a

new nest

floor over the

parasite's eggs,

or

evicting

the

eggs

(Rothstein 1975).

On

the other

hand,

species

that have

not

evolved

with

brood

parasites

almost al-

ways

lack effective defense

mechanisms

and are

extremely

vulnerable

when

they

come

into contact

with brood

parasites

(Mayfield 1965, Rothstein 1975).

There

has

been

recent

concern

over

declining

populations

of

many

forest

song-

birds

in

the

deciduous forest

biome

of

eastern

North America

(Ambuel

and

Tem-

ple

1982,

Temple

and

Temple

1976,

Rob-

bins

1979,

Whitcomb

et

al.

1981),

and it

has

been

suggested

that

intense

brood

parasitism

of

these

naive

hosts

by

brown-

headed

cowbirds,

a

relatively

recent addi-

tion to the avifauna

of

the

biome,

has

played

a role

in the

declines

(Ambuel

and

Temple

1982,

Mayfield

1977a,

Robbins

1979, Whitcomb

et

al.

1981).

We

have

reviewed

the

available

information

on

cowbirds

in the

eastern deciduous

forest

and have identified ecological factors that

determine

the rate of

parasitism

on forest

songbirds.

We have found

temporal,

spa-

tial,

and biogeographic

correlations that

implicate

brood

parasitism

in declines of

forest

songbirds.

CHANGES

N COWBIRD

DISTRIBUTION

ND

ABUNDANCE

Prior

to

the

1800s the brown-headed

cowbird was found

primarily

in the

plains

and

prairies

west of the

Mississip-

pi

River. It was absent from the

large

tracts of unbroken forest that covered

much of eastern North America because

its

feeding

habits and social behavior

restricted

it

to

open

habitat.

As the

east-

ern

forests

were cut

by

settlers and land

was

cleared for

farming,

open

habitat

was

created,

which

provided opportuni-

ties

for

an

eastward

expansion

of the

cowbirds'

range

(Friedmann 1929,

May-

field

1965).

By

the late

1800s,

the cow-

bird was

apparently

widespread

in east-

ern North

America,

but it was

not

abundant

and

was

found

primarily

in

cultivated

areas.

It was

uncommon in

forest habitat

(Bendire

1895,

Friedmann

1929),

and

at that

time,

the

impact

of

cowbird parasitism on the reproductive

success

of

forest songbirds

was

probably

minimal.

In recent

decades

the

cowbird

has

increased

sharply

in

abundance

(Bys-

track

and

Robbins

1977,

Mayfield

1965),

but

its

range

in

eastern North

America

has

probably

not

changed

dramatically.

We examined

regional

Audubon Christ-

mas

bird count

records

from 1900

to

1980

to

document

changes

in

cowbird

abun-

dance in the East within

this

century.

This

method

has

been

used for other

species

(Bock

and

Lepthien

1976a,b).

We examined records for 11 southern

states

(below

370

latitude

from Texas to

the

Atlantic

Ocean)

where most cow-

birds

in the East

migrate

to

spend

the

winter

(Peterson

1980,

Robbins et

al.

1966).

For each

year

we tallied

the

indi-

vidual Christmas

bird

counts

in these

11

states and

calculated

the percentage

of

these counts on

which cowbirds

were

reported.

The

number

of counts

ranged

from 1

in 1900 to

205 in 1980.

Cowbirds

have been

reported

with

increasing

fre-

quency

from

1900

to 1980

(Figure

1),

and

we

interpret

this

as

an indication

that

cowbird numbers have also been steadily

increasing.

This

increase

in

the

abundance of

brown-headed

cowbirds

is most

likely

due

to an

increase

in

their

winter food

supply

and

wintering

habitat.

Waste

grain

in rice

(Oryza

sativa)

fields

in

southern

states

provides

an abundant

source of

food

during

the

winter

months;

this

waste

rice

is the

principal

winter

food

for

several

species

of

blackbirds,

including

the cowbird

(Meanley

1971).

Brittingham

nd

Temple

are

with

the

Department

f

Wildlife

Ecology,

University

of

Wisconsin-Madison,

226 Russell

Laboratories,

630Linden

Drive,

Madi-

son,

WI

53706.

?

1983American nstituteof

Biolog-

ical Sciences. All

rights

reserved.

January

1983

31

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3/6

80

60

o

20

140

-

0

Figure

1. An index

to cowbird abundance

from 1900-1980 taken from

Audubon

Christ-

mas

bird

count records

(r

=

0.961,

P

50%

open.

Our

study

area

contained

more

than 15 such

openings.

We

determined

the location

of nests and

distances

to

openings

by

using compass-

es

and

referring

to aerial

photographs

taken

in October

1980.

We

grouped

nests

into four

categories

depending

on

the distance

from

the nest

to the nearest opening in the forest great-

er than

0.2

ha

in area

(Table

1).

The

percentage

of

parasitized

nests

differed

significantly

among

the four

distance cat-

egories

because

of a decline

in

frequency

of

parasitism

as

distance to

open

habitat

increased.

This decline

does not

appear

to be

the

result

of cowbird's

preference

for

certain host

species

that

nest

only

near

openings.

Rather,

a

similar

array

of

forest-dwelling

songbirds

nested

and

were

parasitized

in

each of

the four

dis-

tance

categories

(Table 2).

The

mean

number of cowbird

eggs per

nest also differed significantly in the four

distance

categories,

but the

mean

num-

bers of

cowbird

eggs per parasitized

nest

did

not

(Table

1).

Therefore,

the decline

in number

of cowbird

eggs per

nest

with

increasing

distance

from

an

opening

is

due to

a decline

in the

percentage

of

nests

parasitized

rather

than to

a lower

number

of

parasitized

nests

with multi-

Table

2. Locations

of

nests

with

respect

to nearest

forest

opening

>

0.02

ha.

Parasitized

nests/total

nests

within

indicated distance from an

opening

Species

0-99

m

100-199

m

200-299

m

?300

m

Acadian

lycatcher

(Empidonax

irescens)

3/8

1/14

1/7

0/8

Least

flycatcher

(Empidonax

minimus)

1/2

0/3

0/0

0/0

Wood

hrush

(Hylocichla

mustelina)

4/6

4/4

1/1

3/4

Veery

(Hylocichla

uscescens)

3/3

0/0

0/1

0/1

Red-eyed

vireo

(Vireo

olivaceus)

0/0

1/1

0/0

0/0

American

redstart

(Setophaga

ruticilla)

0/3

1/1

0/0

0/0

Hooded warbler

(Wilsonia

citrina)

3/3

1/1

0/0

0/0

Mourning

warbler

(Oporornis

hiladelphia)

1/1

0/0

0/0

0/0

Louisiana

waterthrush

(Seiurus

motacilla)

0/0

0/0

0/0

0/1

Ovenbird

(Seiurus

aurocapillus)

5/5

6/8

1/1

0/1

Scarlet

tanager

(Piranga

olivacea)

0/0

1/2

0/0

0/0

Indigo

bunting

(Passerina

cyanea)

6/8

2/2

0/0

0/0

Rose-breasted

rosbeak

(Pheuticus

udovicianus)

0/1

0/1

1/1

0/2

'Arlo

Raim,

University

of

Illinois,

Champaign,

per-

sonal

communication,

October,

1982.

January

1983

33



5/6

pie

cowbird

eggs.

Since the

rate of

cow-

bird

parasitism

is

a direct

function of

cowbird

density

(McGeen

1972,

May-

field

1977a),

the

decline

in

intensity

of

brood

parasitism

in the

interior of

an

extensive

forest

is in

agreement

with

our

data on cowbird

distribution

(Figure

2).

We also

examined the

influence of

very

small

openings

(-0.01

ha and


6/6

Herik

and the Wisconsin

chapter

of

the

Nature

Conservancy

for

permission

to

work in the Baraboo Hills

study

area.

John

Cary

and

Kurt

Johnson were

valu-

able field assistants. We thank

Stephen

Rothstein for comments on an earlier

draft of this

paper

and the staff and

students of the

Department

of Wildlife

Ecology

for

their

continual advice and

support.

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55:

1017-1025.

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