Species: Molothrus ater

This is an obligate brood parasite; females always deposit their eggs in the nests of other bird species,usually resulting in the death of some or all of the host species eggs or nestlings. Females never build a nest, incubate eggs, or tend young. Eggs have been found in the nests of more than 200 species, and young cowbirds are known to have been raised by at least 139 host species (Eastzer et al. 1980).<br><br>Females usually locate nests to parasitize by watching nest-building activities from a perch (Thompson and Gottfried 1976, 1981), especially in semi-open or open habitats. Females have also been observed walking on the ground in dense woods, quietly searching for activity or nests and noisily flapping through dense shrubbery, supposedly to flush incubating females (Norman and Robertson 1975).<br><br>Egg laying, in the nests of other bird species, generally extends from April to July, with most eggs laid in May-June. Individual females lay up to a few dozen eggs each season, but usually much fewer (Jackson and Roby 1992, Holford and Roby 1993). Females may lay eggs in different nests or lay multiple eggs in a single nest. They often, but not always, remove and eat one of the host eggs so that the clutch size is the same when the host female returns. The incubation period (11-12 days, sometimes 10) is often shorter than that of the host species. The host birds feed and tend the nestling and recently fledged cowbirds. A nestling cowbird may weigh up to 10 g when the host nestlings, weighing 1-3 g, hatch and the smaller nestlings are often crushed, crowded out of the nest, or starved (Friedmann 1929, 1963; Mayfield 1965). Cowbird nestlings stay in the nest 10-12 days, begin feeding themselves around day 20-22, and become independent when 25-39 days old (Woodward 1983). Young cowbirds just out of the nest are fed more by the host parent than an equivalent mass of host young. During this period, cowbird young perch at the same height and have the same home ranges as their host (Woodward 1983). Hatching success is not necessarily higher than that of their host species. In Pennsylvania, 57.3% of the eggs of several passerine hosts in unparasitized nests hatched but only 42.6% of the cowbird eggs hatched (Friedmann 1963). Approximately 15% of all cowbird eggs laid in Kansas grasslands resulted in a cowbird young leaving the nest (Zimmerman 1983). Cowbird survival in the nest is similar to host nestling survival but, in Vancouver, cowbird survival was much lower after leaving the nest, possibly because the young cowbird's loud begging calls may attract predators (Smith 1981).