Brown-headed Cowbird
Molothrus ater
褐头牛鹂
Introduction
The brown-headed cowbird 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, while northern populations migrate to the southern United States and Mexico in winter, returning around March or April. The species inhabits open or semiopen country and historically followed bison herds across prairies. Its most distinctive traits are its brood parasitism—laying eggs in the nests of other small passerine birds—and its foraging behavior, which involves following grazing animals to catch insects stirred up by the larger animals. The species' population expanded significantly following European settlement due to forest clearing and the introduction of new grazing animals across North America.
Description
This species has a typical icterid shape but is distinguished by its finch-like head and beak and smaller size. The adult male has iridescent black plumage with a brown head. The adult female is slightly smaller and displays dull grey coloration with a pale throat and very fine streaking on the underparts. Total body length ranges from 16 to 22 centimeters, with an average wingspan of 36 centimeters. Body mass varies from 30 to 60 grams, with females averaging 38.8 grams and males averaging 49 grams.
Distribution & Habitat
The species ranges across temperate and subtropical North America, occupying open or semiopen country. Northern populations migrate to the southern United States and Mexico for winter, returning to breeding grounds by March or April. These birds often travel in flocks, sometimes mixed with red-winged blackbirds, bobolinks, common grackles, or European starlings. Originally following bison herds across prairies, the species expanded its range with forest clearing and the introduction of grazing animals by settlers. They are now common visitors at suburban birdfeeders.
Behavior & Ecology
The diet consists primarily of seeds and insects, with berries consumed rarely. These birds forage on the ground, often following horses and cattle to capture insects stirred up by the larger animals. As an obligate brood parasite, females lay eggs in the nests of other small passerine species, particularly those that build cup-like nests. Cowbird eggs have been documented in at least 220 host species, and more than 140 bird species are known to have raised young cowbirds. Females can lay up to 40 eggs per season. Host responses vary: some abandon nests, others bury the foreign egg, and the brown thrasher physically ejects cowbird eggs. Cowbirds periodically check their eggs, and if an egg is removed, they may ransack the host's nest in retaliatory 'mafia behavior' or destroy nests in 'farming behavior' to force new construction. Young cowbirds develop species-typical behaviors despite being raised by other species.
Conservation
The brown-headed cowbird is not considered threatened or endangered. While it expanded its range with European settlement, control programs targeting this species have been implemented to protect host species impacted by brood parasitism. However, research on Bell's vireo nests suggests these removal programs may have unintended consequences: with fewer cowbirds present, fewer parasitized nests are deserted, resulting in greater nest success for the remaining cowbirds.
Culture
The English name 'cowbird,' first recorded in 1839, refers to this species' tendency to associate with cattle. Due to its brood parasitic behavior and expanded range, the brown-headed cowbird is often regarded as a pest. People sometimes engage in control programs intending to protect species negatively impacted by cowbird parasitism.
Source: Wikipedia · CC BY-SA 3.0
Taxonomy
- Order
- Passeriformes
- Family
- Icteridae
- Genus
- Molothrus
- eBird Code
- bnhcow
Subspecies (3)
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Molothrus ater artemisiae
interior western Canada and western USA
-
Molothrus ater ater
breeds central and east-central USA; winters to Gulf Coast, Florida, and southern Mexico
-
Molothrus ater obscurus
coastal Alaska, western Canada, western USA, and northwestern Mexico
Data Sources
Species description from Wikipedia, licensed under CC BY-SA 3.0.
Taxonomy data from AviList 2025.