Molothrus ater

This species has a heavy conical bill and a brown iris. Adult male has a plain brown head, neck, and upper chest, with the rest of the plumage glossy greenish black. Adult female is gray-brown above, paler below. Juvenile is gray-brown, usually with a finely streaked breast, with pale edgings on the feathers on the back; young males molting to adult plumage are patchy black, brown, and buff. Length is about 7.5 inches (19 cm).

EGGS: vary from pure white to bluish-white; entire egg is covered with chocolate brown or yellowish-tan specks or blotches, which are often heaviest around the larger end (Friedmann 1929). Eggs average 21.5 by 16.4 mm (Bent 1958), with a mass of 3.2-3.4 g (Walkinshaw 1983).

Breeding habitat includes woodland, forest (primarily deciduous), forest edge, city parks, suburban gardens, farms, and ranches. Cowbirds often are associated with forest-field edge habitat and clearings in forests. Feedlots, pastures, and fields with livestock also attract cowbirds, especially in predominately forested areas. In the Sierra Nevada of California, cowbirds feed in horse corrals, meadows with herds of cattle, and at bird feeders in towns, and few cowbirds are found more than 10 km from these food sources (Verner and Ritter 1983). In this same area, the number of cowbirds in meadows decreases as the distance of the meadow from pack stations and horse corrals increases. In migration and winter, cowbirds often occur in open situations, cultivated lands, fields, pastures, and scrub.

Before European settlement of the eastern U.S., Brown-headed Cowbirds were apparently limited to the open grasslands of the United States, mostly west of the Mississippi River. However, because the cowbirds need suitable perches for searching for nests to parasitize, the species was probably limited in the breeding season to prairie riparian corridors (Mayfield 1965).

The main reason for cowbird expansion outside of the Great Plains appears to be the fragmentation of once unbroken forest tracts into small pieces of forest interspersed with fields, pastures, roads, and towns (Mayfield 1977b, Kerlinger and Doremus 1981).

Females hold and defend territories, whereas males have home ranges (0.4-2.5 ha in Ontario; Darley 1982) but do not defend a territory (Ankney and Scott 1982). Breeding and feeding areas of breeding females may be up to several kilometers apart: mean of 1.2 km in Illinois-Missouri; mean of 4.0 km in one area in California (Thompson 1994), and up to 6.7 km in the Sierra Nevada (Rothstein et al. 1980, 1984). NON-BREEDING: Roosts and forages in mixed flocks with red-wing blackbirds (AGELAIUS PHOENICEUS) and grackles (QUISCALUS QUISCULA).

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).

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).

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).