REGIONAL VARIATION IN COWBIRD PARASITISM OF WOOD THRUSHES

Population declines of Neotropical migrant songbirds breeding in the e astern deciduous forest have been attributed, in part, to low reproduc tive success resulting from high rates of brood parasitism by Brown-he aded Cowbirds (Molothrus ater). Wood Thrush (Hylocichla mustelina) nes t records from the Cornell Laboratory of Ornithology were used to test whether rates of parasitism and the number of cowbird eggs or young p er 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 di ffered significantly (P < 0.001) among regions and displayed similar r egional 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 rel ative abundance of cowbirds and negatively cor-related (r = -0.70, P = 0.001) with relative abundance of Wood Thrush. In the Midwest, relati ve abundance of cowbirds was significantly higher and Wood Thrushes si gnificantly lower than in the other two regions. Because of the high a bundance of cowbirds, high percentage of nests parasitized, and high n umber of cowbird eggs per parasitized nest, the effects of cowbird par asitism are particularly severe in the Midwest.