Brown-headed cowbirds practice brood parasitism - Albuquerque Journal

Brown-headed cowbirds practice brood parasitism

Cathryn Cunningham/Journal

Brown-headed cowbirds are a native bird species that can be seen throughout much of North America, including New Mexico.

This cowbird is 7.5 inches long with a short stout bill. The male has a glossy blackish body with a dark brown head. The female is light brownish overall.

The cowbird gets its name from their association with grazing livestock which unknowingly flush up insects with their hooves that the cowbird eats. Prior to cattle, the cowbird associated with bison on the Great Plains for the same reason.

The cowbird’s habitat was confined to the western Great Plains. Clearing and fragmenting forests for human development and agriculture has allowed the cowbird to expand its range eastward. The cowbird avoids heavily forested areas and is most commonly found in open grasslands, pastures, woodland edges and residential areas.

The brown-headed cowbird forages on the ground. Its foods include seeds from grasses, insects and grains. They will visit bird feeders and prefer white millet.

Female cowbirds require a large calcium intake due to laying up to seven eggs. Sources of calcium include, the consumption of snail shells, and of the egg shells from other birds’ nests they have raided.

Brown-headed cowbirds are unique in that they use a nesting strategy called brood parasitism. The female does not build a nest but instead lays her eggs (one to seven) in the nests of other unsuspecting bird species. The female cowbird finds a host nest by watching other birds building nests. The foster parents do all the work of incubating and feeding the cowbird young often at the expense of their own young.

Cowbird young develop faster than many other birds thus demanding most of the available food from the foster parents. I had a Bullock’s oriole family visit my yard and the parents were feeding two young orioles and a much larger, aggressively begging young cowbird.

Some species of birds recognize the cowbird eggs and discard them. The yellow warbler is one species that recognizes cowbird eggs, but is too small to remove them so instead builds another nest on top of the old one, hoping a cowbird does not discover the new nest.

It is easy to condemn the cowbird for this behavior, but it is important to realize that the cowbird has adapted to do this. It is mainly human development practices that have allowed the cowbird to expand its range eastward thus negatively affecting a wider variety of species through brood parasitism.

Mary Schmauss is the owner of Wild Birds Unlimited in Albuquerque. A lifelong birder and author of “For the Birds: A Month-by-Month Guide to Attracting Birds to your Backyard.”

 

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