Portal de Periódicos da CAPES

REGIONAL VARIATION IN COWBIRD PARASITISM OF WOOD THRUSHES

1993; Wilson Ornithological Society; Volume: 105; Issue: 2 Linguagem: Inglês

1938-5447

Jeffrey P. Hoover, Margaret C. Brittingham,

Animal Ecology and Behavior Studies

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 (Molothm 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 para- sitism 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. Received I7 April 1992, accepted 22 Oct. 1992. Neotropical migrant songbirds breeding within the eastern deciduous forest have undergone apparent population declines since the late 1940s (Robbins 1979, Whitcomb et al. 198 1, Askins et al. 1990). One suggested cause of the declines has been low reproductive success resulting from high rates of brood parasitism by Brown-headed Cowbirds (Molothrus ater) (Gates and Gysel 1978, Brittingham and Temple 1983, Robinson 1988). In addition, the cowbird has been implicated in the near extinction of the Kirtland's Warbler (Dendroica kirtlandii) and other rare species (Rothstein 1975, Mayfield 1977). Although cowbird parasitism appears to have a large effect on repro- ductive success of forest songbirds in the Midwest (Brittingham and Tem- ple 1983, Robinson 1988) the magnitude of the problem for songbirds nesting throughout the eastern deciduous forest is unknown. Because the historic range of the cowbird was west of the Mississippi River, and because cowbirds are associated ecologically with open habitat (Mayfield 1965, Dufty 1982), their abundance and impact on forest songbirds may be highest close to this historic range and in regions with more open habitat. As a result, rates of cowbird parasitism and the importance of cowbird parasitism as a factor responsible for declines of forest songbirds