Cowbirds outsource parenting to other species, but an innate password tells their children to copy cowbird songs.
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.
"We kind of opened the paper with this existential question," said Sarah
London, a neuroscientist at the University of Chicago. "How do you know
who you are if no one's shown you who you are?"
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."
Females don't sing, but they have a simple "chatter call" that
develops normally regardless of what a female hears growing up. They use
it in a variety of contexts, including immediately after hearing a song
they like. Because the chatter call is innate and is often paired with
songs, the researchers suspected it might function as a password to help
young cowbirds learn.
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."
Next, the researchers examined which genes were active in the birds'
auditory forebrains, the part of the brain responsible for processing
and learning songs. Compared to the cowbirds trained with mourning dove
coos, the birds trained with chatter calls showed increased activity in
genes associated with neural plasticity. This suggests the chatter calls
helped shift the birds' brains into a malleable state that allowed them
to learn, said Louder.
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.
The gene expression analysis suggested that even though the females
had heard both canary songs many times, they only learned to recognize
the song that was paired with chatter calls during training, said
Louder. The findings were published today in the journal Current Biology.
"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.
While the study only examined the chatter call in the context of song
learning, Louder suspects it might serve as a more general cue for
species recognition, helping young birds to find other cowbirds after
leaving the nest. A similar mechanism could also be at play in many
different species, said Louder. Most animals are raised by their own
kind, but they could still benefit from cues that help them figure out
what to learn. There are hints that a whistled note at the start of the
golden-crowned sparrow's song may serve a password-like function.
And passwords don't necessarily have to be song-related. They could be
something an animal sees or feels, and they could shape virtually any
learned behavior.
"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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