The brown-headed cowbird (Molothrus ater) is a small, obligate brood parasitic icterid native to temperate and subtropical North America. It is a permanent resident in the southern parts of its range; northern birds migrate to the southern United States and Mexico in winter, returning to their summer habitat around March or April.
Taxonomy
The brown-headed cowbird was described by the French polymath Georges-Louis Leclerc, Comte de Buffon in 1775 in his Histoire Naturelle des Oiseaux from a specimen collected in the Carolinas. The bird was also illustrated in a hand-colored plate engraved by François-Nicolas Martinet in the Planches Enluminées D'Histoire Naturelle, which was produced under the supervision of Edme-Louis Daubenton to accompany Buffon's text. Neither the plate caption nor Buffon's description included a scientific name, but in 1783, Dutch naturalist Pieter Boddaert coined the binomial name Oriolus ater in his catalogue of the Planches Enluminées. The brown-headed cowbird is now placed in the genus Molothrus that was introduced by English naturalist William John Swainson in 1832 with the brown-headed cowbird as the type species. The genus name combines the Ancient Greek mōlos meaning "struggle" or "battle" with thrōskō meaning "to sire" or "to impregnate". The specific name ater is Latin for "dull black". The English name "cowbird", first recorded in 1839, refers to this species often being seen near cattle.
Three subspecies are recognised: M. a. artemisiae Grinnell, 1909 – interior west Canada and west USA M. a. obscurus (Gmelin, JF, 1789) – coastal Alaska, Canada, USA, and northwest Mexico M. a. ater (Boddaert, 1783) – southeast Canada, east and central USA, and northeast Mexico
Description
The brown-headed cowbird is typical for an icterid in general shape, but is distinguished by its finch-like head and beak and smaller size. The adult male is iridescent black in color with a brown head. The adult female is slightly smaller and is dull grey with a pale throat and very fine streaking on the underparts. Their total length is and the average wingspan is . Body mass can range from , with females averaging against the males' average of .
Distribution and habitat
The species lives in open or semiopen country, and often travels in flocks, sometimes mixed with red-winged blackbirds (particularly in spring) and bobolinks (particularly in fall), as well as common grackles or European starlings. These birds forage on the ground, often following grazing animals such as horses and cattle to catch insects stirred up by the larger animals. They mainly eat seeds and insects.
Before European settlement, brown-headed cowbirds followed bison herds across the prairies. Their population expanded with the clearing of forested areas and the introduction of new grazing animals by settlers across North America. They are now commonly seen at suburban birdfeeders.
In 2012, brown-headed cowbirds in northwest Riverside County, CA tested positive for West Nile Virus.
Behavior and ecology
Brood parasitism
The brown-headed cowbird is an obligate brood parasite; it lays its eggs in the nests of other small passerines (perching birds), particularly those that build cup-like nests. The brown-headed cowbird eggs have been documented in nests of at least 220 host species, including hummingbirds and raptors. More than 140 different species of birds are known to have raised young cowbirds. The young cowbird is fed by the host parents at the expense of their own young. Brown-headed cowbird females can lay up to 36 eggs in a season.
Host response
Some host species, such as the house finch, feed their young a vegetarian diet. This is unsuitable for young brown-headed cowbirds, meaning few survive to fledge. Accepting a cowbird egg and rearing a cowbird chick can be costly to a host species. In the American redstart, nests parasitized by cowbirds were found to have a higher rate of predation, likely due in part to the loud begging calls by the cowbird nestling, but also partly explained by the fact that nests likely to be parasitized are also more likely to be preyed upon.