Brown-headed Cowbird (Molothrus ater)

[order] PASSERIFORMES | [family] Icteridae | [latin] Molothrus ater | [UK] Brown-headed Cowbird | [FR] Vacher a tete brune | [DE] Braunkopf-Kuhstarling | [ES] | [NL] Bruinkopkoevogel

Subspecies

Physical charateristics

Nearly 10% larger than Starling, with deeper-based, rather short, noticeably conical bill, and often raised tail. Quite small but robust Nearctic icterid, with relatively dark and uniform plumage. Male dusky-black, with umber-brown hood extending to mantle and breast. Female and immature dusky mouse-brown, with paler throat.

Listen to the sound of Brown-headed Cowbird

[audio:http://www.aviflevoland.nl/sounddb/B/Brown-headed Cowbird.mp3]

Copyright remark: Most sounds derived from xeno-canto

Range

North America, Middle America : South Canada to South Mexico

Habitat

In temperate to subtropical Nearctic, linked with prairies and pastures through ancient adaptation to feeding with bison, and later cattle and horses, which stir up insect food. Range enlarged by modern forest clearance. Habit of brood-parasitism has, however, led to attachment to woodland edges, thickets, and places where low or scattered trees are interspersed with grassland.

Reproduction

Brown-headed Cowbirds have unusual breeding behavior: they never build nests or raise their own young. Males typically arrive on the breeding grounds before the females. Pairs are generally monogamous. Females lay eggs in other birds’ nests and leave the rearing to other species. They find nests to parasitize by looking for birds building nests, either by walking along the ground, perching quietly in shrubs or trees, or making noisy flights back and forth, possibly to flush potential hosts. The female generally chooses an open cup-nest to parasitize, and usually lays one egg per nest. She waits to lay the egg until the host bird has at least one egg in its nest, and often removes one egg from the nest before laying her own. She continues the process over a period of about a month and can lay up to 40 eggs a season. Incubation time is short, 10 to 12 days, which allows the young cowbird to get a head start in the nest. Young cowbirds grow rapidly, giving them a competitive advantage over the other young in the nest. Young cowbirds usually leave the nest after 8 to 13 days, but are not fully independent from the hosts until they are about 25 to 39 days old. Then they typically form small flocks with other juveniles. Over 220 species have been observed with Brown-headed Cowbird eggs in their nests, and at least 144 species have raised Brown-headed Cowbird young to the fledgling stage, often at the expense of their own young.

Feeding habits

During the summer, about half of the Brown-headed Cowbirds’ diet is made up of seeds, and the other half consists of insects and other invertebrates. Their winter diet is made up of more than 90% seeds and waste grain. Females also eat eggshells from the nests where they lay their eggs and mollusk shells, especially during the breeding season. This practice may provide the calcium they need to produce the large number of eggs they lay.

Conservation

This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). Despite the fact that the population trend appears to be decreasing, the decline is not believed to be sufficiently rapid to approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is extremely large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.
Breeds in North America from British Columbia and southern Mackenzie east to central Quebec and southern Newfoundland, south throughout USA to central Mexico.
Accidental. Britain: male, Islay (Strathclyde), April 1988. Norway: female, Jomfru island (Telemark), June 1987.

Migration

Status varies from fully migratory (populations breeding north of 40 degrees N in west, and north of 45 degrees N in east), through partly migratory, to sedentary (most birds breeding south of 35 degrees N). Winters in southern and eastern North America, north to north-central California, New Mexico, Kansas, southern Great Lakes region, and Nova Scotia, and south to southern Mexico. Wintering north of snow-line has increased greatly since 1950, especially in north-east.

Distribution map