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(Inside Science) — Most songbirds learn to sing by copying songs they hear around them. But young brown-headed cowbirds face a problem: they aren’t raised by their own kind. Female cowbirds lay their eggs in the nests of more than 100 different kinds of birds, foisting the work of chick-rearing onto unwitting foster-parents.
Now, a new paper describes how the cowbird chicks may learn to recognize and sing their own species’ songs.
London and her colleagues, including Matthew Louder of the University of Tokyo and Mark Hauber of the University of Illinois at Urbana-Champaign, found that part of the answer appears to be a “password” — a simple call that the birds know innately. This password activates learning mechanisms in young cowbirds’ brains, prompting them to remember other vocalizations they hear at the same time.
Males raised in isolation will develop something that resembles a cowbird song, but with important differences. In the wild, young males change their developing songs to match the songs of other cowbirds in their vicinity, leading to regional differences or “accents.”
To test how chatter calls affect song learning, Louder and his team collected baby cowbirds from their hosts’ nests and raised them by hand. When the male nestlings were 70-80 days old — old enough that they would likely have joined flocks of other cowbirds if they were in the wild — the researchers began playing them pairs of vocalizations: first a song, then a chatter call.
Instead of using recordings of cowbird songs, the researchers trained the birds on canary songs. This allowed them to isolate the effect of a recorded cowbird chatter call without worrying about any other instincts a cowbird song might trigger. As a control, they exposed other young males to a canary song followed by a mourning dove’s coo.
Sure enough, the males that heard chatter calls paired with the canary songs began sounding significantly more canary-like, incorporating distinctive canary whistles into their repertoire.
“It doesn’t sound like a canary. No canary would be fooled. But the chatter call is definitely doing something,” said Louder. “It looked to us and sounded to us like they were trying to mimic the canary song.”
Finally, the researchers turned their attention to females. Female cowbirds may not need to sing, but they still have to learn which songs belong to desirable mates of their own species. Unlike the previous experiments where males were trained with either a dove coo or a chatter call, each female was trained with two different canary songs, one that was always paired with a chatter call and one was always paired with a mourning dove coo.
Right before testing the females’ brains, the researchers played them one last song — either the one previously paired with the chatter call, or the one that had been paired with the dove coo. Then, they looked for activity in a set of genes associated with novel experiences.
“I think that it’s a really great paper,” said Stephen Nowicki, a biologist at Duke University in Durham, North Carolina, who was not involved in the research but co-authored a commentary article about it in the same journal. “It’s really pretty convincing evidence that this sound, the chatter, is acting as a password.”
David White, an animal behaviorist at Wilfrid Laurier University in Canada, said he was initially skeptical of the password hypothesis when Hauber first proposed it nearly two decades ago. But he said the new study, combined with some of White’s own findings on the importance of the chatter call, has finally persuaded him that Hauber was right.
“It explains a big thing that’s bothered people for a while about cowbirds,” said Louder. But, he said, “I don’t think it’s just a cowbird thing.”
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