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. 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 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: Body mass can range from , with females averaging against the males' average of .

Distribution and habitat

thumb|right|Brown-headed cowbird male (right) courting female

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.

In 2012, brown-headed cowbirds in northwest Riverside County, CA tested positive for West Nile Virus.

Behavior and ecology

right|thumb|Juvenile in California

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. The young cowbird is fed by the host parents at the expense of their own young. Brown-headed cowbird females can lay up to 40 eggs in a season.

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Image:Eastern Phoebe-nest-Brown-headed-Cowbird-egg.jpg|Eastern phoebe nest with one brown-headed cowbird egg

File:Brood parasitism cowbird yellowthroat.jpg|Common yellowthroat feeding juvenile brown-headed cowbird

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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.

Unlike the common cuckoo, the brown-headed cowbird is not divided into gentes whose eggs imitate those of a particular host. Host species sometimes notice the cowbird egg, with different hosts reacting to the egg in different ways. Some, like the blue-grey gnatcatcher, abandon their nest, losing their own eggs as well. Others, like the American yellow warbler, bury the foreign egg under nest material, where it perishes. The brown thrasher physically ejects the egg from the nest. Brown-headed cowbird nestlings are also sometimes expelled from the nest. Nestlings of host species can also alter their behavior in response to the presence of a cowbird nestling.

Parasite response

Song sparrow nestlings in parasitized nests alter their vocalizations in frequency and amplitude so that they resemble the cowbird nestling, and these nestlings tend to be fed equally often as nestlings in unparasitized nests.

Brown-headed cowbirds seem to periodically check on their eggs and young after they have deposited them. Removal of the parasitic egg may trigger a retaliatory reaction termed "mafia behavior". According to one study <!-- by the Florida Museum of Natural History published in 1983, --> the cowbird returned to ransack the nests of a range of host species 56% of the time when their egg was removed. In addition, the cowbird also destroyed nests in a type of "farming behavior" to force the hosts to build new ones. The cowbirds then laid their eggs in the new nests 85% of the time.

Male behavior and reproductive success

Social behaviors of cowbird males include aggressive, competitive singing bouts with other males and pair bonding and monogamy with females.

In lab experiments, brown-headed cowbirds which are exposed to more complex social environments have greater reproductive success

Relationship to humans

With the expansion of its range and its parasitic behavior, the brown-headed cowbird is often regarded as a pest. People sometimes engage in cowbird control programs, with the intention of protecting species negatively impacted by the cowbirds' brood parasitism. A study of nests of Bell's vireo highlighted a potential limitation of these control programs, demonstrating that removal of cowbirds from a site may create an unintended consequence of increasing cowbird productivity on that site, because with fewer cowbirds, fewer parasitized nests are deserted, resulting in greater nest success for cowbirds.

References

  • Brown-headed cowbird Information at Animal Diversity Web
  • Brown-headed cowbird - Molothrus ater—USGS Patuxent Bird Identification InfoCenter