Brown-headed Cowbirds occur in grasslands with low and scattered trees as well as woodland edges, brushy thickets, prairies, fields, pastures, orchards, and residential areas. Brown-headed Cowbirds generally avoid forests. Development and fragmentation of forests in the eastern United States have allowed Brown-headed Cowbirds to greatly expand their range eastward. In winter, Brown-headed Cowbirds roost along with several species of blackbirds in flocks numbering more than 100,000 birds.Back to top
Brown-headed Cowbirds feed mostly on seeds from grasses and weeds, with some crop grains. Insects such as grasshoppers and beetles, often caught as cows and horses stir them into movement, make up about a quarter of a cowbird’s diet. As you might imagine, female cowbirds have a large calcium requirement from laying so many eggs. To satisfy it, they eat snail shells and sometimes eggs taken from nests they’ve visited.Back to top
Cowbirds lay eggs in a great variety of nests, including Red-winged Blackbird nests in marshes, dome-shaped Ovenbird nests on the forest floor, cup nests in shrubs and treetops, and even occasionally in nests in tree cavities. Over 140 host species of the Brown-headed Cowbird have been described, from birds as small as kinglets to as large as meadowlarks. Common hosts include the Yellow Warbler, Song and Chipping sparrows, Red-eyed Vireo, Eastern and Spotted towhees, and Red-winged Blackbird.
Doesn’t build its own nest. Experiments done with artificial nests in an aviary suggest that Brown-headed Cowbirds tend to choose nests containing eggs of smaller volume than their own.
|Clutch Size:||1-7 eggs|
|Egg Length:||0.7-1.0 in (1.8-2.5 cm)|
|Egg Width:||0.6-0.7 in (1.5-1.8 cm)|
|Incubation Period:||10-12 days|
|Nestling Period:||8-13 days|
|Egg Description:||White to grayish-white with brown or gray spots.|
|Condition at Hatching:||Naked except for sparse tufts of down, eyes closed, clumsy. Cowbird chicks tend to grow faster than their nestmates, allowing them to get more attention and food from their foster parents.|
Brown-headed Cowbirds usually forage on the ground in mixed flocks of blackbirds, grackles, and starlings. They get their name from their close association with grazing livestock (and formerly bison), which flush up insects for the birds to eat. Cowbirds fly directly, with constant wingbeats. When males sing, they often raise their back and chest feathers, lift their wings and spread their tail feathers, and then bow forward. Groups of males may do this together. Female Brown-headed Cowbirds don’t build a nest or rear young. They find nests by watching quietly for signs of other birds building nests, or they flutter through vegetation trying to flush birds from their nests. When young cowbirds hatch, they may roll the other eggs out of the nest.Back to top
Brown-headed Cowbirds are common across most of North America, although populations slightly declined between 1966 and 2019, according to the North American Breeding Bird Survey. Partners in Flight estimates a global breeding population of 130 million and rates them 7 out of 20 on the Continental Concern Score, indicating a species of low conservation concern. Originally a bird that followed bison of the Great Plains, the Brown-headed Cowbird spread eastward in the 1800s as forests were cleared. The Brown-headed Cowbird's habit of nest parasitism can threaten species with small populations, such as the endangered Kirtland's Warbler and Black-capped Vireo.Back to top
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