Brownheaded cowbird

Molothrus ater


Oriolus ater Boddaert, 1783, Carolina. Three geographically discrete forms are recognized.


French: Vacher à tête brune; German: Braunkopf-Kuhstarling; Spanish: Vaquero Cabecicafé, Tordo Negro.


6.5-7.5 in (16.5-18 cm); female 1.1-1.8 oz (30.5-51 g), male 1.1-2 oz (32.5-58 g). Sexually dimorphic in color. Males black with a brown head; the black is greenish-glossed, and purple-glossed on the neck. Females are entirely brown, with the throat somewhat paler. Juveniles resemble females, but have scaly backs and boldly-streaked underparts. The bill is short and conical.


Breeds from central and northeastern British Columbia, Alberta, central Saskatchewan and southern Manitoba, east through central Ontario and Quebec to Newfoundland, and south to the Isthmus of Tehuntepec, Mexico. Resident from Nova Scotia, Maine, Illinois, eastern Kansas, west across Oklahoma, Texas, southern New Mexico, and Arizona, and along the Pacific coast north to southwestern British Columbia; does not breed in southern Florida, on the Gulf lowlands of eastern Mexico, or the lowlands of southwestern Mexico. Winters along the Gulf of Mexico coast of Mexico, and on the Pacific coast from Jalisco south to the Isthmus of Tehuntepec, and in southern Florida.


Open woodlands and deciduous forest edge; in migration and winter in open areas, cultivated lands, fields, pastures, and scrub.


Males display with a full "song-spread" display. In some populations, where they are monogamous, males guard their mates. Females lay their eggs in the nests of other species of birds. They usually do this early in the morning, and remove one of the host's eggs, replacing it with one of theirs. In winter, found in flocks that usually contain several different species of blackbirds as well as European starlings (Sturnus vulgaris).


Forage low in vegetation or on the ground, often near the feet of grazing ungulates, where they pick up insects that have been flushed. During the nonbreeding season, they eat primarily grain.


In some areas, males are monogamous; in others they commonly are simultaneously paired to two or more females. No nest is built, but they have been recorded to have parasitized the nests of more than 220 host species (144 of which have been seen to fledge cowbird young). Females do not lay clutches in the usual sense, but one egg is produced each 1-7 days, interrupted by 2 days when no eggs are produced; eggs are laid from March to early-August. Incubation 10-12 days; fledging at 8-13 days.

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