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Regional Variation in Cowbird Parasitism of Wood Thrushes

Jeffrey P. Hoover and Margaret C. Brittingham
The Wilson Bulletin
Vol. 105, No. 2 (Jun., 1993), pp. 228-238
Stable URL: http://www.jstor.org/stable/4163283
Page Count: 11
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Regional Variation in Cowbird Parasitism of Wood Thrushes
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Abstract

Population declines of Neotropical migrant songbirds breeding in the eastern deciduous forest have been attributed, in part, to low reproductive success resulting from high rates of brood parasitism by Brown-headed Cowbirds (Molothrus ater). Wood Thrush (Hylocichla mustelina) nest records from the Cornell Laboratory of Ornithology were used to test whether rates of parasitism and the number of cowbird eggs or young per parasitized nest varied regionally with distance from the historic range of the cowbird. Rates of parasitism differed significantly (P < 0.001) among the Midwest (42.1%), Mid-Atlantic (26.5%), and Northeast (14.7%). Mean number of cowbird eggs or young per parasitized nest differed significantly (P < 0.001) among regions and displayed similar regional trends with means of 2.09, 1.64, and 1.21, respectively. Rates of parasitism were correlated positively (r = 0.64, P = 0.002) with relative abundance of cowbirds and negatively correlated (r = -0.70, P = 0.001) with relative abundance of Wood Thrush. In the Midwest, relative abundance of cowbirds was significantly higher and Wood Thrushes significantly lower than in the other two regions. Because of the high abundance of cowbirds, high percentage of nests parasitized, and high number of cowbird eggs per parasitized nest, the effects of cowbird parasitism are particularly severe in the Midwest.

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