Archive for October 20th, 2010

Cowbirds: Once Drifters, Now Home Wreckers

Wednesday, October 20th, 2010

This is Passport to Texas

The brown-headed cowbird migrated with bison across the Great Plains. Because it’s hard to raise a family on the road, cowbirds laid their eggs in other nests; host birds unwittingly raised their young.

The problem with the cowbird eggs is that normally they’re big eggs; they hatch earlier than the host eggs do; and they’re very vocal and hungry and beg for food.

Biologist, Marsha May, says the cowbird hatchlings out compete the host’s offspring for food and starve them out, putting a dent in the population of that species. Back when bison roamed, cowbirds didn’t have quite the same impact.

Black-capped vireos, which are an endangered species now, evolved where if they were parasitized by a brown-headed cowbird, they would leave that nest and re-nest – start a new nest. Well, if the cowbirds had already moved through, that would have worked.

Without bison, cowbirds hang with cows. Because cows are fenced in and don’t migrate, neither do cowbirds.

They’re parasitizing all the birds in that area – their nests – and they’re having a major impact on some species like the black capped vireo, because the black-capped vireo keeps re-nesting and that’s wasting a lot of energy, and if it’s constantly being parasitized, then no young will be reproduced at all that year.

We’ll have more on cowbirds tomorrow.

That’s our show… we receive support from the Sport Fish and Wildlife Restoration Program… funded by your purchase of fishing and hunting equipment and motor boat fuels.

For Texas Parks and Wildlife…I’m Cecilia Nasti.