Brown-Headed Cowbird Parasitism of Lazuli Buntings; Relationships with Habitats and Ungulate Hosts

Abstract

Brown-headed Cowbirds Molothrus ater, a brood parasite that lays its eggs in the nests of other birds, has recently undergone a tremendous range expansion. Before European settlement, this species was restricted to short-grass prairie, where it followed buffalo Bison bison and fed on insects stirred up by their movements (Lowther 1993). Settlement of North America by Europeans, the subsequent large-scale deforestation, and extirpation of buffalo lead to Brown-headed Cowbirds shifting to associate with cows and horses. These changes in landscape and host associations allowed a rapid range expansion and increase in numbers (Payne 1977, Laymon 1987, Rothstein et al.1980). Brown-headed Cowbirds now are found from the Pacific to the Atlantic coast, and from south-central Mexico north to tree line in Canada (Lowther 1993). Cowbirds are apparently expanding their range altitudinally as well, so that they can now be found breeding over 3,000 m in elevation (Hanka 1985). Lazuli Buntings Passerina amoena are small neotropical migrant birds that breed throughout western United States and southwestern Canada. These conspicuous birds breed in a wide variety of brushy habitats, ranging from sea level along the Pacific coast to over 3,000 m in Sierras and Rocky Mountains. Preferred breeding habitat includes arid bushy hillsides, riparian habitats, wooded valleys, aspen, willow, alder or cottonwood thickets, sage brush, chaparral, open scrub, recent post-fire habitats, thickets and hedges along agricultural fields, and residential gardens (Greene et al. in press).