Ecology and behavior of cowbirds and their impact on host populations

Cowbirds of the genus Molothrus are brood parasites that lay their eggs in the nests of other (host) species. Because many hosts raise cowbird young instead of their own, brood parasitism can substantially reduce host breeding productivity, and high levels of brood parasitism can threaten some host populations. Population declines and the endangered status of several species have been linked to high levels of cowbird parasitism. Neotropical migrant landbirds may be particularly vulnerable to cowbird parasitism, especially in landscapes where cowbird feeding habitat has been created or enhanced by human activities and cowbird numbers are high. The purpose of this chapter is to review the natural history of North American cowbirds with special emphasis on the brown-headed cowbird, their effects on host population dynamics, and to identify the areas where research is most needed.