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Why do brown-headed cowbirds move down south towards mexico?

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Why do brown-headed cowbirds move down south towards mexico?

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Habitat

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

Food

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

Nesting

Nest Placement

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.

Nest Description

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.

Nesting Facts

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.

Behavior

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

Conservation

Brown-headed Cowbird are common across most of North America, although populations slightly declined by about 31% between 1966 and 2014, according to the North American Breeding Bird Survey. Partners in Flight estimates a global breeding population of 120 million, with 77% spending some part of the year in the U.S., 14% in Canada, and 31% in Mexico. They rate an 7 out of 20 on the Continental Concern Score and are not on the 2014 State of the Birds Watch List. Originally a bison-following bird 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

Backyard Tips

Even though Brown-headed Cowbirds are native to North America, many people consider them a nuisance bird, since they destroy the eggs and young of smaller songbirds and have been implicated in the decline of several endangered species, including Kirtland's Warbler and Black-capped Vireo. Cowbirds often flock with other species of blackbirds, and they may come to your yard if it contains open ground or lawn, or if you scatter grain for ground birds. If your yard is large enough to keep livestock, there's a good chance you will find cowbirds there.

This species often comes to bird feeders. Find out more about what this bird likes to eat and what feeder is best by using the Project FeederWatch Common Feeder Birds bird list.

Credits

Dunne, P. (2006). Pete Dunne's essential field guide companion. Houghton Mifflin Harcourt, New York, USA.

Ehrlich, P. R., D. S. Dobkin, and D. Wheye (1988). The Birder's Handbook. A Field Guide to the Natural History of North American Birds, Including All Species That Regularly Breed North of Mexico. Simon and Schuster Inc., New York, USA.

Lowther, Peter E. (1993). Brown-headed Cowbird ( Molothrus ater), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.

Lutmerding, J. A. and A. S. Love. Longevity records of North American birds. Version 2015.2. Patuxent Wildlife Research Center, Bird Banding Laboratory 2015.

North American Bird Conservation Initiative. (2014). The State of the Birds 2014 Report. US Department of Interior, Washington, DC, USA.

Partners in Flight (2017). Avian Conservation Assessment Database. 2017.

Sauer, J. R., J. E. Hines, J. E. Fallon, K. L. Pardieck, Jr. Ziolkowski, D. J. and W. A. Link. The North American Breeding Bird Survey, results and analysis 1966-2013 (Version 1.30.15). USGS Patuxtent Wildlife Research Center (2014b). Available from http://www.mbr-pwrc.usgs.gov/bbs/.

Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.

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Brown-headed Cowbird Life History, All About Birds, Cornell Lab of Ornithology

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The Brown-headed Cowbird is a stocky blackbird with a fascinating approach to raising its young. Females forgo building nests and instead put all their energy into producing eggs, sometimes more than three dozen a summer. These they lay in the nests of other birds, abandoning their young to foster parents, usually at the expense of at least some of the host's own chicks.

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How long do The Brown-headed cowbird live for?

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How long do The Brown-headed cowbird live for?

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Where do Brown-headed cowbirds come from?

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Where do Brown-headed cowbirds come from?

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The Brown-headed Cowbird comes from North America. In the winter the Brown-headed cowbird moves down south towards mexico