Research by West and her colleagues revealed that during their first winter, juvenile cowbirds learn from adult cowbirds proper courtship and other behaviors that enable them to successfully mate in the spring.
West and colleagues also gathered experimental data that revealed a surprising discovery: If cowbirds are housed with canaries during the first winter of their lives, then by spring the cowbirds will think they are canaries. They will make sexual advances at other canaries and sing canary songs.
Hauber, the Berkeley behavioral ecologist, says the finding underscores how important it is that cowbirds find their flock once they gain independence from their foster parents. "If you socialize with the wrong species during that first winter, you'll be confused for life," he said.
Hauber's "password hypothesis" holds that cowbirds and other parasitic birds possess a simple behavioral trait or cue that is both species specific and recognized by young birds that have never before encountered birds of their ilk.
The behavioral ecologist and his colleagues recognized such a trait in a specific cowbird call known as the chatter call.
Through a host of field and laboratory experiments, the researchers found that six-day-old and two-month-old naive cowbirds are attracted to the chatters produced uniquely by male and female adult cowbirds.
The researchers are uncertain how the cowbirds learn the secret password in the first place. But the scientists suspect that the begging call the cowbirds use to get food from their foster parents is acoustically similar to the chatter call.
"Indeed, as cowbird chicks grow older, their begging call morphs into a chatterlike call," Hauber said. "Perhaps listening to herself or himself, the young cowbird can identify others of their own kind."
According to West, attraction to chatter calls, as well as other visual cues that may also serve as passwords, may all help with recognition.
Parasitic Pests
Today cowbirds, which are native to the midwestern U.S., are considered a nuisance implicated in the decline of several species of songbirds throughout North America.
"They are attracted to short grass. They are ground-dwelling species looking for seeds and insects," Hauber said. Thus, where there are cattle, horses, bison, and lawn mowers, there are likely to be cowbirds, he added.
Cutting forests for pasture and suburban sprawl has allowed cowbirds to spread throughout the U.S. They are known to parasitize at least 200 different bird species, many of which have evolved no resistance to the parasitic birds, Hauber said.
Cowbirds are larger than most of the species they parasitize. As a result, when cowbird eggs hatch, the fledgling chicks monopolize the food brought by the foster parents, often to the detriment of the nestlings of the host species.
Hauber concedes that cowbirds are a nuisance species but noted that the birds tend to be scapegoats for the decline of some North American bird species. "It's … habitat disappearance that is the crucial factor that seems to have allowed cowbirds to enter the picture," he said.
Regardless, he said, the study of cowbirds "may provide different lessons about the way evolution proceeds."
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