Only cowbirds sing the watery blues

Cowbirds are special among songbirds for the “watery” timbre of their singing, which resembles the sound of falling droplets striking water, a quick burst followed by a fading ripple.

University of Utah biologist Franz Goller and colleagues investigated how cowbirds pull off this vocal trick and discovered the answer is surprisingly complex.

Birds produce sound using the syrinx, a vocal organ with two separate sound sources—one on the left and one on the right. To create the gurgling, bubbling quality in their songs, cowbirds rapidly switch between these two sides, according to new research led by Goller, a professor emeritus of biology.

But the researchers discovered the watery effect is also associated with the way these birds control their breathing.

“What's striking about cowbird syrinx is that the left pair of vocal folds is much bigger than the right pair. One idea is that the high frequencies are made on the right side only. The portions of these introductory notes that have the watery timbre are alternations of left and right contributions,” said Goller, who has long studied birds’ vocalizations with funding from the National Institutes of Health. “We were able to pinpoint the exact mechanism by which this sound coloration is generated.”

His findings were reported last month in the journal Current Biology.

Listen to the sounds brown-headed cowbirds make.

A biologist, a physicist and a psychologist walk into a lab....

Goller’s interdisciplinary research team, which included a physicist and psychologist, focused their study on brown-headed cowbirds, Molothrus ater, one of six species in its genus and the one common in Utah. They also studied starlings that they raised with cowbirds in an experiment to see how they imitate cowbird songs.

Cowbirds belong to the Icteridae family, whose other members, such as blackbirds, can also produce sounds with a unusual timbre—a musical term that refers to the character or feel of a note as opposed to its pitch.

“Typically, when we talk about timbre, we often think of the upper vocal track, so above the sound source. There are filter qualities that we use in speech all the time. That you recognize a voice of an individual is based exactly on these filter qualities,” Goller said. “But a lot of sound coloration features are also generated at the source, and we know very little about that. Here we managed to study in detail how this watery quality of cowbird song comes about through rather complex interactions between respiration and cowbirds' two sound sources.”

What exactly is timbre?

Timbre is a powerful component of the sounds animals and humans produce, but it is not well understood, according to co-author Brenton Cooper, a neuroscientist who studies speech and language.

“You can hear the difference between an ‘A’ and an ‘I.’ The perceptual difference between those two sounds is largely timbre. The frequency is pretty much the same, the amplitude can be the same, yet you hear those two vowels very differently,” Cooper said.

The timbre of cowbird song is best characterized as droplets falling on water. This sound comes in two stages.

“The first sound is the water hitting the surface, and the second, more audible sound is 50 milliseconds later when an air bubble implodes and different size water droplets will generate different frequencies like strings on a guitar,” said Cooper, now a professor of psychology at Texas Christian University. After completing his Ph.D. in psychology at the University of Utah, Cooper joined Goller’s lab as a postdoctoral researcher in the early 2000s.

The researchers had long been intrigued with cowbirds’ tendency to hold their breath, which turns out to be a key element behind the timbric qualities of their singing. They generate strong pressure in the syrinx while not producing sounds.

“It's not normal. If birds are generating a positive expiratory pressure, they're generating sound the whole time,” Cooper said. According to the research findings, cowbirds briefly close their syrinx while building up air pressure, then release it suddenly to produce a splash-like burst of sound.

Precision airflow and a double voice box

“What that allows is what we call rapid attack, a really rapid onset of sound production that mimics the striking of the water droplet onto the surface of water, and then that slow decay. The two alternating sound sources then give you this perception of different water droplet sounds because different size water droplets generate different frequencies.”

The team conducted experiments with captive starling hatchlings that were raised with cowbirds in the laboratory, which raised the young birds.  These starlings learned to replicate their cage mates’ unique vocalizations using the same unusual breathing-and-valve control observed in cowbirds.

“Being raised next to a cowbird is what gave them the auditory experience so they could memorize that song and reproduce it,” Cooper said. “Birds are vocal learners and they learn just like we learn human speech. That's what got me into this. It's the best animal model system for understanding human speech and language.”

The study shows that timbre is shaped by intricate interactions between physiology and motor control. And birds, it turns out, are even more sophisticated sound engineers than previously known.

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The study, titled “Water-like timbral quality in birdsong arises from complex motor control of two sound sources,” appears in the December edition of Current Biology. The third co-author is Goller’s long-time collaborator Gabriel B. Mindlin, a professor of physics at the University of Buenos Aires.