Brown-Headed Cowbird

Molothrus ater

Illustration by H. Douglas Pratt

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The common brown-headed cowbird is the most widespread brood parasite in North America. Polytypic. Length 7.5" (19 cm).

Identification A smallish, compact and stocky blackbird with a short and thick-based bill, almost finchlike. Foraging birds are commonly seen on the ground with the tail cocked. Adult male: the glossy black body, with a greenish iridescence, contrasts with a brown head. The eyes are dark; the bill and legs are black. Adult female: dull brownish throughout, with darker wings and tail. The secondaries show crisp pale fringes. The face has a beady-eyed look due to dark eyes; the lores are pale, as is the area below the eyes. The whitish throat contrasts with the darker face; the underparts are obscurely streaked. The bill is dark, but shows a pale or horn-colored base to the lower mandible; the legs are black. Juvenile: it resembles the female, but is more strongly streaked below, and upperparts are often scaly looking due to pale feather fringes.

Geographic Variation The 3 subspecies, differing mainly in size and darkness of females, are not field identifiable.

Similar Species The male’s combination of glossy black body and brown head is diagnostic. The female is extremely similar to the vagrant shiny cowbird female. However, the brown-headed is paler overall, showing a whitish throat and a pale face with a beady-eyed look. The shiny has a more marked dark eye line and paler supercilium, giving it a more striking face pattern. Compared to a shiny, the brown-headed is more compact, with a shorter tail and bill. The brown-headed’s dark bill shows a pale or horn base to the lower mandible; on the shiny, the bill is shiny black. On the closed wings the secondaries show obvious pale fringes on the brown-headed. Finally, the brown-headed has a longer and more pointed wing, and the outermost primary (P9) is equal in length or longer than the second outermost primary (P8).

Voice Call: a soft kek. Females give a distinctive dry chatter, while males may give a single modulated whistle, particularly just after taking off. Song: primary song is a series of liquid, purring, gurgles followed by a high whistle, bub ko lum tseeee or glug glug glee. The song of the brown-headed has the highest frequency range of any species in North America. Males also give a flight whistle, which may be considered a secondary song rather than a call. It is a geographically variable series of 2 to 5 whistles, often frequency modulated. The flight whistle is given both in flight and while perched. Note that primary songs appear to be hardwired, while flight whistles are learned; this accounts for why “dialects” are found in the latter but not in the former.

Status and Distribution Common. Breeding: open and edge habitats. Migration: a short-distance diurnal migrant; northbound mostly mid-March–mid-April, southbound late July–October. Winter: open sites, agricultural areas, feeders.

Population The brown-headed cowbird increased its range and population greatly during the early 1800s, when the eastern forests were cleared. Commonly it is said that the brown-headed was restricted to the Great Plains, where the buffalo herds were, and they spread east and west from there. However, there are multiple subspecies, suggesting the range of the species was more widespread and primarily that an expansion in abundance occurred. More recently, brown-headed cowbird numbers have been on a decline.

—From the National Geographic book Complete Birds of North America, 2006

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