The Importance of the TPWD Cowbird Trapping Program

by Ray Kirkwood on May 10, 2012

By Mike Mauldin & Wilfred Korth, Certified Texas Master Naturalists
Photos by Mike Mauldin

Some Mid-Coast Chapter of Texas Master Naturalists participate in a Brown-headed Cowbird Trapping Program in several locations along the Texas coast, under the direction of the Texas Parks and Wildlife Department.

The Brown-headed cowbird is a member of the blackbird family. Females are pale brown with grey-brown heads. The male is metallic black with a coffee-brown head. Both are about 7.5 inches long. Conical beaks are perfect for their primary foods: grasshoppers and seeds. The female lays her egg in another bird’s nest, usually the nest of a songbird. 

Why are we trapping Brown-headed cowbirds?

Female Brown-headed Cowbird

The brown-headed cowbird evolved with roaming buffalo across the great prairies, and in order to survive, it had to follow the buffalo. It could not build its own nest, but had to lay eggs in another bird’s nest and hope the “host” birds would raise its young. Brown-headed cowbirds have been called “passerine chickens” because their laying cycle adapted to take advantage of a continuous supply of host nests for a 2-month period, making them able to lay new eggs frequently during this period. The female brown-head can lay 40 or more eggs per season. The Brown-headed chick hatches before the songbird’s eggs, so it is larger than the host bird’s hatchlings, gets more than its share of the food, and grows faster. Often the host bird’s hatchlings don’t survive from lack of food or from being pushed out of the nest.

But when man killed off all the buffalo, fenced in all the prairies, and created herds of non-roaming cattle, the cowbird adapted to this new livestock and lifestyle. They did not, however, change their nesting style, and that created the problem we have today. Instead of moving elsewhere, they now stay with the fenced-in cattle and lay their 40 or so eggs in the local area, with the potential to decimate the populations of birds that are hosting their eggs. Cowbirds are the only nest-parasitic birds in North America—they continue to rely solely on other birds to raise their young. And the nest-parasitism is working very well—the Brown-headed cowbird range has spread from the prairies of North America to all of the lower 48 States in our Union. Even if only 3% of the hatched Brown-heads survive, they can double their population every 8 years due to the size of their population and number of eggs laid each year.

Cowbirds contribute to the decline of Songbirds

There is a decline in the population of many songbirds, and in particular of a few already threatened species like the Black-capped vireo, Golden-cheeked warbler, and Kirtland’s warbler. Nest parasitism by cowbirds may be a

Male Brown-headed Cowbirds

significant factor contributing to the declining numbers of these and other songbirds in North America. Researchers have discovered that 225 bird species are being parasitized by the Brown-head. That’s why we trap cowbirds—trapping programs began across North America to help reduce the impact on our threatened songbirds.

 

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