Birds flex muscles to hit high notes, biologists say

SALT LAKE CITY - University of Utah scientists describe songbirds as "vocal gymnasts," some capable of staggering tonal ranges and trilling 100 times faster than a blink of a human eye.

One such musical athlete is the male zebra finch, which university biologists are studying to determine how it learns songs and why the male can cover the four octaves on the top half of a piano keyboard, while the female is capable of only low-pitch bursts.

A study published June 29 discovered that this gender difference, known in science as "sexual dimorphism," is the result of male finches' more powerful muscles in its syrinx, songbirds' counterpart to the human larynx, instead of just air pressure pushing out the lungs.

"When we started we already knew the (male) muscles were larger. How do you translate that into sound? That's the key," said Tobias Riede, a research assistant professor of biology affiliated with the university-administered National Center for Voice and Speech. The activity of the syrinx muscles is the more significant variable for the birds' ability to regulate pitch, according to results published by the Public Library of Science journal PLoS ONE.

Scientists interested in songbirds, or oscines, study zebra finches, natives of Australia, because they are musically active year-round and they display extreme dimorphism.

The birds also are readily available, reproduce easily in captivity and learn their songs quickly, according to biology professor Gary Rose, who studies how birds learn songs. Learned vocalization, a signature human trait, is rare in nature and studying it in non-humans could shed light on the neuro-physiological basis for language.

"We're the only primate that learns our acoustic communication signals," said Rose, who was not involved with the latest study. "We have a period of time (as children) where we have to hear a particular language in order to reproduce it later. That's not true in primates and other mammals, with the exception of cetaceans (whales and dolphins), which you can't study the way you can study birds."

The study builds on past research by university biologist Franz Goller, a co-author of the new paper who is exploring the neuroscience behind bird song. His past work helped establish that birds are not born with the ability to sing particular songs, but must learn them.

"A baby robin that is isolated will not develop a proper song, like a human infant needs his parents to learn speech," Goller said at a recent presentation. Singing is key to songbirds' reproductive success, he said, but scientists have yet to figure out how songs both attract mates and protect territory.

"You can't tell which is the sexy guy from listening to the song," Goller said. "What is it about that song that tells females, 'I'm the one you want to have babies with' and tells rivals, 'If you get close I'll beat you up' ?"

The research team included John Fisher, of the department of physiology. An electronics engineer, Fisher developed the tiny back-mounted wireless transmission system the biologists used to record air pressure in the finches' air sacs.

The scientists began by recording songs from six males and six females for two weeks. They then implanted tubes, each containing a sensor to measure air pressure, into one of the four sacs that drives air through the lungs and syrinx. Fisher's wireless recording system allowed the birds to fly and sing freely during the experiments.

Next, the scientists surgically severed the nerves that control the syrinx, and their songs were again recorded. This time they sounded different, with the males dropping their pitch, proving muscle control was central to tonal control.

Riede suspects singing may have helped drive the last 50 million years of bird evolution. About half of the world's 10,000 bird species are capable of song.

Birds that can deploy muscles as well as air pressure for song have an evolutionary edge because they can carry a wider range of sounds and can better adapt to changing environments, Riede said.

Higher-frequency notes are better able to pierce the din of background noises, although low-pitch sounds carry farther. And songs with a variety of pitches and volumes stand a better chance of holding an audience, and thereby giving the singer a better chance of passing on his genes.