HILO, HAWAII--Cowbird moms that place their eggs in forest foster homes make a better choice than those that seek foster homes in fields, according to research presented here at the Society for Conservation Biology meeting on 30 July. The finding could help explain why cowbirds are such a scourge for forest-dwelling songbirds, which run themselves ragged trying to feed baby cowbirds.
The brown-headed cowbird is a so-called brood parasite that lays its eggs in other birds' nests, thereby letting other species do all the heavy lifting of chick rearing. Once limited to the Great Plains--a huge field, basically--cowbirds have expanded their range in the past 300 years. Humans have fragmented forests with fields, giving cowbirds access to habitats they otherwise couldn't penetrate--and apparently hastening the decline of forest songbirds, which can't rear their own chicks as successfully while trying to stuff the gaping maws of cowbird chicks.
To see where cowbirds reproduce most successfully, conservation biologist Rachael Winfree of Princeton University in New Jersey compared nests in an old field to those in a patch of forest. She tracked the fate of 452 nests of 25 species in the Shawnee National Forest in Illinois. Female cowbirds were better off laying their eggs in forest nests: Cowbirds that laid eggs in fields averaged only five fledglings, she estimated, whereas those that laid eggs in forest birds' nests ended up with 12.7 fledglings. Winfree suspects more baby birds are lost to predators in fields than in forests.
The research could help explain why cowbirds lay their eggs in now-accessible forest birds' nests, even though they once were limited to field habitats. Frank Thompson, a wildlife biologist with the U.S. Forest Service in Columbia, Missouri, says of Winfree's work, "It's important to understand why cowbirds prefer certain habitats to breed in--and this is going to help do that."