Nest Site Characteristics Related to Cowbird Parasitism and Predation in the Indigo Bunting

Jesse Howell and Sarah L. Martin

Department of Biology, University of Kentuckyjessehowell@uky.edu

Introduction

Methods

Results

Discussion

Indigo buntings (Passerina cyanea) are a shrub-nesting passerine which suffer high rates of brown-headed cowbird (Molothrus ater) parasitism and predation. Both nest parasitism and predation are known to be influenced by nest habitat and landscape. For instance, the edge effect hypothesis predicts nesting near an edge may lead to increased predation since predators commonly follow corridors1. Our study’s objective was to determine which nest site characteristics influenced the probability of parasitism and predation, and to determine if there is a relationship between nesting near edges and increased rates of cowbird parasitism and/or predation.

• We examined nests at Griffith Woods in Harrison County, Kentucky from May – August 2008. • Nests were monitored every two days for development. • Nest fate was determined to be parasitized only, predated only, parasitized and then predated, or successful (fledged at least one bunting). • Nests with known fate (N = 57) were noted with the following nest site characteristics:

• nest height • observer objective nest visibility (%) at 1 m from nest in the four cardinal directions and above the nest• observer objective average shrub cover (%) averaged from four quadrants extending from the nest in a 5 m radius• distance from nest to nearest edge • type of edge (abrupt v. gradual) • width of edge

Brood parasitism was affected by several nest site characteristics, while predation was not. This suggests that cowbirds may be targeting nests based on particular nest site criteria. In our study, this was specifically those nests which were higher, less visible from above, and in less dense shrub cover 5 m around the nest. Predators, on the other hand, were diverse and likely influenced by human-made paths to nests; therefore, predation appears to have occurred randomly. We found no support for the edge effect hypothesis on rates of predation or parasitism at the nest. A similar study on habitat disturbance effects in buntings also found that edge characteristics did not affect rates of parasitism2. These results may have further implications for conservation, since a variety of vegetation features other than edge effects must be managed in order to reduce cowbird parasitism and predation of passerine nests. Literature Cited:

1. Manolis, James C., Andersen, David E., Cuthbert, Francesca J. 2002. Edge effect on nesting success of ground nesting birds near regenerating clearcuts in a forest dominated landscape. The Auk 119:955-970.

2. Suarez, Andrew V., Pfening, Karin S., Robinson, Scott K. 1997. Nesting success of a disturbance-dependent songbird on different kinds of edges. Conservation Biology 11:928-935.

Nest Selection

Edge EffectParasitism •Type of edge did not affect the rate of parasitism (χ2

1 = 1.55, P = 2.14).• Edge width was also an insignificant factor concerning the rate of parasitism. However, distance from the nest to the nearest edge demonstrated an influential trend (Figure 3).

Parasitism• Nest height (Figure 1), nest visibility from 1 m above and average shrub cover in a 5 m radius independently influenced the probability of parasitism occurring at the nest (Figure 2).

Predation• There were no nest site characteristics that significantly influenced the probability of predation.

Figure 2. Comparison of nest visibility from above (effect = -2.60, Wald chi-square= 5.80, P = 0.01) and average shrub cover (effect = - 2.23, Wald chi-square= 3.86, P = 0.05) on parasitism. (*) indicates P < 0.05.

Predation• Edge type was an insignificant indicator of rate of predation upon the nest (χ2

1 = 0.05, P = 0.82). • Using separate logistic regressions, edge width and distance from edge were insignificant factors relating to predation (Figure 4).

Female brown-headed

cowbird

Figure 1. Comparison of nest height (effect = - 0.04, Wald chi-square = 5.56, P = 0.02) on parasitism. (*) indicates P < 0.05.

Parasitism

Nes

t Hei

ght (

cm)

Figure 3. Comparison of distance from nest to nearest edge (effect = - 0.001, Wald chi-square = 3.34, P = 0.06) and edge width (effect = - 0.0007, Wald chi-square = 0.04, P = 0.84 on parasitism.

cm

Figure 4. Comparison of distance to edge from nest (effect = 0.0003, Wald chi-square = 0.89, P = 0.35) and edge width (effect = 0.0001, Wald chi-square = 0.13, P = 0.72) on predation. Common nest predators included black rat snakes and raccoons.

cm