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Male Birds' ability to learn Song affects
Female Mating Response


Researchers have found that how well a male songbird learns his
song affects the female's mating response. The first evidence that
female birds use song-learning ability as an indicator of male quality.
The study goes beyond previous such studies, which have only
demonstrated that very poor or absent male songs affect female
mating response.


According to the scientists, the finding offers broader insight
into the role that traits learned by males play in mating
success.

In an article in the September 22, 2002, Proceedings of the Royal
Society of London: Biological Sciences (now online), biologists
led by Duke University Professor of Biology Stephen Nowicki
reported studies in which they tested the mating response of
female song sparrows to songs of captive-raised males.
Importantly, the scientists had analyzed the males' songs in
detail to determine the degree of accuracy with which the males
copied songs they attempted to learn. They found that the females
preferred those songs that came closest to wild-type songs they
heard when young and presumably learned as models. The
scientists' research was sponsored by the National Science
Foundation.

According to Nowicki, he and his colleagues in the field have
long theorized that female songbirds pay attention to male song
as an indicator of fitness.

"We've developed experimental evidence that there is a link
between early stress, male brain development and song-learning,"
he said.

"But until now, experimental and field observations showing that
females were interested in song only contrasted the presence or
absence of song, or relatively gross features of song, like the
size of the repertoire. This is the first study to explicitly
demonstrate that females care about song-learning quality," he
said.

To test the effects of fine differences in song quality on female
response, the researchers trained captive-reared male song
sparrows to sing by exposing them to the recorded songs of wild
birds. To induce variation in stress among the birds, some were
placed on a restricted diet during development. Using
spectrographic analysis, the researchers rated the captive-reared
birds on two measures of song quality

-- how much of the wild-bird song they copied versus how much
they invented, a practice common among song sparrows. Those birds
who did invent more song elements also tended not to copy well
those elements they did copy.

-- how close the males had come to actually matching just the
wild-bird song elements they were attempting to copy


To determine the effects of song quality, the researchers exposed
wild-caught adult females -- presumably experienced in listening
to male songs -- to the captive-reared males' songs. The
scientists measured female response to the songs by the amount
they performed characteristic and distinctive female mating
presentation display -- which includes a shivering of the wings,
the lifting of the tail and a characteristic call.

As a control, the scientists exposed the wild-caught females to
what the scientists had judged as well-learned male songs, as
well as the digitally recorded wild songs. The female birds
responded equally to both.

However, when the scientists exposed the females to the
captive-reared males' songs, they found the females responded
more strongly to male songs that had been better learned by both
of the scientists' measures.

"The females showed a strong preference for songs that had been
copied well, as opposed to songs that had been copied poorly,"
said Nowicki. "And by our measures, the males got points taken
off for originality. That seems to make sense because we would
argue that males that deviate from original song haven't learned
the song as well." In addition to insight into bird song, said
Nowicki, such studies can give basic insight into the evolution
of animal signals in general.

"We know mating selection is a very powerful evolutionary force
that has led to phenomena such as the evolution of extravagant
displays and the evolution of size differences between genders. I
believe that this work demonstrates that mating selection might
not be acting directly on the obvious trait that is expressed,
but on the mechanisms that underlie the expression of that trait.
In the case of bird song, a male's song reflects the birds'
developmental history, and song expression is only the trait that
the female can gain access to for information about -- in this
case -- brain mechanisms."

Also, said Nowicki, the discovery that females assess song
quality emphasizes the importance of studying the neurobiology of
song expression and placing it in an evolutionary context.

While the current studies show clearly that females prefer
well-learned songs, among the next research steps, said Nowicki,
will be to determine how females learn to judge song quality.

"There is only very thin evidence that females learn song, so
it's a major scientific question whether females are learning
something about the population that they're living in, and using
that as a way of assessing males," he said. Such female studies
also will reveal whether the female's ability to distinguish good
songs from bad reflects the birds' fitness and influences
evolution, said Nowicki.

This story has been adapted from a news release issued by Duke
University.


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