Cowbirds

A brown-headed cowbird (By Alan Vernon/CC BY-NC-SA 2.0)

It was the opposite of Rachel Carson’s “Silent Spring.” Avian ecologist Andrew Cox had spent a lot of time in Missouri’s woods and fields filming bird nests for his doctoral thesis, and songbird chicks seemed to be doing better than expected. He also noticed that brown-headed cowbirds, which lay their eggs in songbirds’ nest, were doing so less often than earlier reports suggested. A question came to mind: Could the two be related?

A new study in the journal PLoS ONE coauthored by Cox, now a postdoctoral researcher at the University of Missouri, reports that as cowbird populations shrank over the last 20 years, songbird nests produced increasing numbers of young.

Cowbirds depend on leaving their eggs in other species’ nests, which the adoptive parents then care for as their own, often to their own chicks’ detriment. The cowbirds are not picky, leaving their eggs in the nests of more than 200 other types of birds. “They are a wickedly clever species,” says Cox.

Not only do cowbirds crowd out their adopted siblings, but they appear to draw predators to the nest, possibly with their loud and frequent squawking for food. Scientists have also filmed cowbirds pushing songbird eggs and baby songbirds out of their nests, presumably to make room for more cowbirds

Cox compared current data on songbird nests with archival data collected over the last two decades from several sources. He chose three songbird species that were well represented in the data: the indigo bunting, the Acadian flycatcher, and the northern cardinal.

The parasitic birds lay their eggs in nests built in non-forested areas much more often than they lay them in nests found in dense woods. Cox hypothesizes that as forest crept over the state, cowbirds declined. He points out that cowbird populations are not shrinking everywhere, and that his study was limited to Missouri.

The species that Cox studied are some of the state’s more abundant songbirds. (He couldn’t find enough data to study the rarer species.) But, he says, his findings could potentially apply to a larger range of species. As long as cowbird populations don't rise, songbirds in Missouri at least might not be too silent.