The Brown-headed Cowbird (Molothrus ater) is an obligate brood parasite that lays eggs in the nests of more than 220 host species (Lowther 1993). Brown-headed Cowbirds typically reduce the reproductive success of their hosts by removing host eggs and producing nestlings that demand extensive care from host parents. Additionally, cowbird eggs require a shorter incubation period than the eggs of many host species (e.g., Briskie and Sealy 1990; Kattan 1995), and incubation of cowbird eggs may disrupt incubation of host eggs (McMaster and Sealy 1998).
To parasitize a nest successfully, cowbirds must lay eggs in a host’s nest when the host female is laying (or shortly thereafter, if cowbird eggs require a shorter incubation period than host eggs). However, some cowbirds also depredate nests much later in the nesting cycle, presumably to induce renesting (Smith and Arcese 1994; Arcese et al. 1996).