To a human ear the songs of all male zebra finches sound more or less the same. But faced with a chorus of this simple song, female finches can pick the performer who sings most beautifully.
Zebra finches are found in Australia, and they usually mate monogamously for life -- making this a high-stakes decision for the female finches. The zebra finch is among about a third of songbirds who learn a single song from their fathers early in life, and sing it over and over, raising the question of how female songbirds distinguish between them to choose a mate.
Scientists believe that most male songbirds evolved to sing a variety of songs to demonstrate their fitness. Under that theory, the fittest songbirds will have more time and energy to work on their vocal stylings -- and attract females with their varied vocal repertoire.
New research using machine learning shows finches may be sticking to one tune, but how they sing it makes a big difference. Published recently in the journal Nature, the study reveals the complexity of a single zebra finch song -- and what female songbirds might be hearing in their prospective mates' seemingly "simple" songs.
LOOKING AT SOUND
When researchers analyze birdsongs, they're often not listening to them -- but rather looking at spectrograms, which are visualizations of audio files.
"So I put together that, 'Hey, what humans are doing is looking at images of these audio files. Can we use machine learning and deep learning to do this?'" said Danyal Alam, the lead author on the new study and a postdoctoral researcher at the University of California San Francisco.
Alam, along with Todd Roberts, an associate professor at UT Southwestern Medical Center and another colleague, used machine learning to analyze hundreds of thousands of zebra finch songs to figure out how they were different from each other and which variations were more attractive to female songbirds.
The researchers found one metric that seemed to get females' attention: the spread of syllables in the song. The females seemed to prefer longer "paths" between syllables. This isn't something humans can easily pick up by listening to the songs or looking at the spectrograms -- but based on how these algorithms mapped the syllables, the researchers were able to see them in a new way.
To check their hypothesis, the researchers took the findings back to the birds.
They generated synthetic bird songs to see if females preferred those with a longer path -- and they did, suggesting the birds' intended audience picked up on the same pattern as the researchers' computers.
Alam and his colleagues also found that baby birds had a harder time learning the long-distance song patterns than the shorter ones -- which suggests fitter birds would be more able to learn them, the researchers said.
SECRETS OF A SIMPLE BIRDSONG
The study's finding is consistent with what has been shown in other species: The more complexity or difficulty in a song, the more appealing it is to female birds.
"A lot of signals in animal communication are meant to be an honest signal of some underlying quality," said Kate Snyder, a researcher at Vanderbilt who wasn't involved in the new paper.
For example, she said, if you look at a peacock, you see the male birds with the longer and more beautiful tails are better at attracting mates. Maintaining a tail like that is expensive for the bird -- so it must be good at finding food and surviving in its environment to have the time to devote to keeping its tail nice.
"Learning takes a lot of time, energy, brain space," Snyder said. Only the fittest male birds will have the time and energy to devote to learning to sing.
Among finches, that work has just been harder to spot -- until now.
"We used to think of this single song repertoire as perhaps a simple behavior," Roberts said. "But what we see is that it's perhaps much more complicated than we previously appreciated."