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Brown-Headed Cowbird
bronzed-headed cowbird
Brown-headed cowbird, photographed at Yellowstone National Park, Wyoming, in June 2017.
Brown-Headed Cowbird
 

Cowbirds might be the worst parents of the avian world. While other bird species will " dump" excess eggs in the nests of others of their kind, only the cowbird relies on it as its sole means of rearing young. Thing is, cowbirds don't nest at all, so they target those of other species, often warblers.

And with devastating results for the hosts. Whole families can be wiped out by the voracious demands of a single hatched cowbird egg. So with that introduction, meet the brown-headed cowbird, Molothrus ater, one of three cowbird species. It's a winter visitor to most of Florida, but can be found year-round in the northern part of the state. Its year-round range extends across much of the eastern and central United States, south into Mexico and along the Pacific Coast to Puget Sound. It's a summer visitor to the rest of the country and a good chunk of Canada.

The theory behind the strategy is simple: rather than spending energy building a nest and raising young, female cowbirds, including the brown-headed, concentrate on producing eggs. Lots of them. As many as three dozen eggs in a single season. A single clutch can have between one and seven eggs, but only one will be dropped in any single nest. Cowbirds will time their dump when the female of the host species is laying her own eggs, minimizing the chance that the imposter will be discovered. The cowbird will remove or damage one of the host's own eggs and replace it with her own. The process is called parasitic brooding. Some species will detect the imposter egg and reject it, either by pecking it or throwing it out of the nest. Some species will detect it but the "foster parents" will be too small to remove the imposter. Most will fall victim to ruse, with deadly consequences for their own kind.

 
 
Brown-Headed Cowbird
 

Brown-headed cowbird eggs require 10 to 12 days of incubation before hatching, and eight to 13 days of care thereafter before fledging. The young cowbirds grow faster and larger than the host's own and demand more food and attention. They'll often roll the host's eggs out of the nest. The parents might be literally worked to death trying to sate the appetite of the demanding interloper. Brown-headed cowbirds have been known to target the nests of as many as 220 species, some of which are endangered. Cowbirds have been seen as contributing to the declining numbers of several, including Kirtland's warbler. Habitat destruction by us humans is also a major factor.

Their strange name comes from their habit of following first buffalo, then domesticated cattle as they graze. The bovines would stir up insects, which the cowbirds would pick off. That also played a role in their strange lifestyle. According to the Cornell Laboratory of Ornithology, it's believed that their need to keep up with roaming herds of buffalo (technically bison) either caused the need to adopt a parasitic approach to rearing young or gave them the freedom to do so. They are birds of open terrain — fields and meadows — and forest edges. Seeds are a major component of their diet.

They are small birds, eight or nine inches long, with a wingspan of more than 14. Males have a brown head and and black, iridescent body. They can be confused with grackles, but their short, triangular bills easily separate the two. Females are dull brown all over. Brown-headed cowbirds are members of Icteridae, the blackbird family.

Photographs by David Sedore

 
     
Links for Brown-Headed Cowbird Cornell Laboratory of Ornithology National Audubon Society National Geographic Society
 
Unless otherwise stated, all photographs are property of the publishers and may not be used without their express permission.