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Master Naturalists: Cowbirds a menace to songbird populations

March 30, 2010 at midnight
Updated March 30, 2010 at 10:31 p.m.

Brown-headed cowbirds are parasitic, laying their eggs in other birds' nests, even killing host birds chicks and destroying their eggs. Endangered songbirds species can be protected through trapping cowbirds on ranches and farms.

By Paul and Mary Meredith

Brown-headed cowbirds and four other less common cowbirds are native to the New World.

The Audubon Society calls all cowbirds "brood-parasitic," making them villains in declining numbers of native songbirds that nest in Texas.

What's caused the problem?

For millennia, insect-eating cowbirds followed massive nomadic herds of bison foraging for grasses and forbs (wildflowers) on the Great Plains. Bison disturbed insects as they moved, and the insects became easy prey for cowbirds.

Cowbirds' feeding was easy; their breeding wasn't. They couldn't stay in one place long enough to build nests, lay and incubate eggs and raise a brood to maturity.

Gradually, they adapted in three ways. First, cowbirds whose eggs hatched faster survived. Today's eggs hatch in 11 to 12 days, a day or two sooner than most smaller songbirds' eggs.

Hatching sooner is important because cowbirds' second adaptation was stopping nesting and becoming nest parasites.

Rather than nest themselves, they learned to observe nesting of birds like songbirds that could "host" their eggs, and began laying their eggs in those nests.

Cowbirds often removed a host's egg or hatched chick from its nest and laid their own egg there. Because cowbird chicks hatched sooner and were generally larger than the hosts' chicks, they got more food from the host parents, who naively incubated them, often causing the hosts' chicks to starve to death.

Third, today's cowbirds are prolific, laying about 40 eggs a season over a two-year lifespan.

However, even with these adaptations, cowbirds' nomadic lifestyle limited their impact on host species' survival.

Host and parasite species were in balance, until Europeans and their domesticated animals appeared.

What's the problem?

When Europeans introduced cattle, slaughtered bison herds, and fenced prairies, the balance was disturbed. Cattle herds grazed on fenced lands.

Feedlots were introduced for fattening them. Cowbirds were no longer nomads.

Consequently, their superb parasitic-nesting behavior now attacks the same population of host birds year after year. About 3 percent of a cowbird's eggs survive, and a female lays about 40 eggs per year. So each female cowbird produces 2.3 adult offspring in her 2-year lifetime.

Now that cowbirds live in the same locations, cowbird chicks are often found in over 90 percent of hosts' nests.

How bad is it?

Brown-headed cowbirds are opportunistic, preying on both treetop and ground-nesting birds.

Reproduction of about 220 species is impacted by cowbirds.

Biologists estimate that one female cowbird will destroy 3.5 host birds in her lifetime. Moreover, in some cases cowbirds are attacking at-risk or endangered species. Among those species are song sparrows, black-capped vireos, golden-cheeked warblers, yellow-breasted chats, Bell's vireos, willow flycatchers and Kirkland's warblers.

Is there a solution?

When nest-parasitism of perching songbirds' nests is high, Texas farmers and ranchers can legally trap and destroy cowbirds on their land, under permits issued by Texas Parks and Wildlife Department. Birds may only be trapped from March 1 through May 31 - or until fledgling birds of other species appear in the traps.

Traps are placed in open areas near herds and away from nesting areas, baited with grain, provided with water and shade and checked daily.

Cowbirds are disposed of using humane, fast and certain means. Non-target birds caught in the trap are released. All captures are recorded and reported confidentially to TPWD.

Does it work? Can I help?

Experiments using cowbird-trapping on contract grazing land in Fort Hood, near Killeen, have shown that it works. Nest-parasitism there was reduced from 90 percent to 6 percent in one year. And in one case, involving 13,000 acres of contract grazing land, follow-up surveys show black-capped vireos are increasing.

In a different case, involving 53,000 acres of contract grazing land, surveys show warblers are increasing.

If you have land, run cattle, and have cowbirds, you may be interested in becoming part of this TPWD program, which was co-developed with the Central Texas Cattlemen's Association.

If you are interested, contact your local TPWD office for further information on regulations, permitting, reporting, trap design; and for free training from TPWD personnel in approved program procedures.

Paul and Mary Meredith are master naturalists. Contact them at paulmary0211@sbcglobal.net.

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