Birds Sing The 'Story Of Their Lives,' Theorize Duke Biologists

May 04, 1998

Note to editors: A photo of Steve Nowicki is available on the University Photography ftp site at ftp://152.3.242.19/pub/ . The filename is Nowicki.
He may be reached at (919) 684-6950, snowicki@duke.edu

DURHAM, N.C. -- When male songbirds sing to attract mates, the quality of their song might directly portray their fitness, say Duke University biologists in advancing a new theory of how birdsong serves as a mating signal. Their theory holds that female birds carefully analyze a male's song quality to judge how well he has overcome the stress of early life and how effective a mate he will be.

The researchers, Associate Professor of Zoology Steve Nowicki and research associates Susan Peters and Jeffrey Podos, discussed their theory in an article in the latest (dated February) issue of American Zoologist. Their research is sponsored by the National Science Foundation.

"This hypothesis has to do with the fact that song is a learned trait," Nowicki said, "And that this learning is supported by specific regions of the brain that develop during a particular period when the young bird is most likely to undergo stress." Such stress, Nowicki said, might consist of the nutritional stress a nestling faces during the rapid growth period songbirds typically face early in life, or perhaps stress resulting from the effects of parasites.

"Males that have more complex songs -- or songs that females prefer -- went through the learning process better, because their brains developed better. It's because they were better nourished or less stressed during this critical period in the late nestling, early fledgling phase."

Thus, said Nowicki, the song quality, in essence, tells the story of a male bird's life, not only how fit he is, but how well his parents raised him.

The Duke zoologists said their theory clarifies a largely unanswered question until now: how exactly does a female bird benefit from choosing a mate based on his song. Early theories had held that the elaborate male bird song, like other exaggerated male mating traits, was an arbitrarily chosen symbolic trait. Such a trait would undergo a runaway hypertrophy as females evolved a preference for the trait, causing a positive feedback in which their increasing preference triggered ever-greater male expression of the trait.

Such traits, often cited, are the showy plumage of male birds, from the male peacock's tail to the brilliant scarlet of the male cardinal. The intricate warbling of male song birds is, indeed, a highly exaggerated trait, Nowicki said.

"Bird song is far more complex than it needs to be for the information it conveys," he said "It can be as complex as human speech by such measures as the number of units per time, bandwidth, or the rate of acoustic modulations. But if a bird is only more or less just advertising its species, defending a territory or attracting females, why then are bird songs so complex?"

Mathematical modeling of bird mating did initially suggest that such runaway hypertrophy of random traits could explain such exaggerated traits, but later studies found this might occur only under relatively limited conditions, Nowicki said.

Later theories did attempt to explain exaggerated male traits as a way for males to offer information about themselves to females. One theory, called the "handicap principle," held that males with the most exaggerated male traits, such as a showy tail or an elaborate song, showed their genetic fitness because they could survive despite the attention-attracting handicap.

However, such theories did not satisfactorily explain the precise mechanism by which bird song functioned as an information source. So, Nowicki and his colleagues began to develop their theory based on the analysis of numerous studies of bird song and bird development.

"We amassed as much evidence as we could about the temporal relationship between nutritional stress, brain development and the timing of song-learning," Nowicki said. "And, we believe we show that this overlap is really quite good, revealing a mechanism by which song quality could be used by females to assess quality of males."

For example, the researchers cited studies showing male birds with superior resistance to disease or parasites withstood insults better and could devote a larger proportion of their energies to ornament, making their selection by a female a wise choice.

Other studies reviewed by the Duke biologists showed that the quality or complexity of bird song did reflect the birds' genetic quality, as reflected by survival and survival of offspring.

Still unclear, said the researchers, is exactly what quality the female is assessing when she critiques a male's song.

"She might be assessing the quality of brain development in general, which will be important for the males to find food, evade predators, find nest sites, navigate around his territory, and so forth," said Nowicki. The female might also be indirectly assessing the male's genetically endowed parenting abilities, since he is the result of his own parents' care. Or, the female might be assessing how well the male overcame the stress of rapid growth that is the lot of all young birds.

Another remaining puzzle is why some song bird species have exceedingly complex songs, while other species with similar life histories have relatively simple songs that still function as sexual selection traits.

"We think the answer is that the female is not necessarily assessing song complexity for its own sake, but as a measure of the quality of song learning," Nowicki said. "For example, the female might assess how precisely the male copied just one element of a song.

"We can consider bird song as a sexually selected trait, but we have to understand that the focus of selection is on the bird's brain."

Nowicki and his colleagues are now conducting laboratory experiments to confirm their theory, in which they feed growing birds differently and study the effects on the complexity of the male's song.

They are also studying the female side of the mating equation, comparing females' reactions to recorded songs of different complexity. Researchers can quantify such responses by assessing how emphatically a female assumes a copulatory posture when she hears a male song. The researchers will also explore how nutrition affects the even-more mysterious learned or inborn ability of females to judge male song.
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Duke University

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