Meet one of the most talented singers on Earth: the hermit thrush (Catharus guttatus). This medium-sized North American songbird has garnered praise from both musicians and ornithologists because its songs sound markedly musical, with trills and slides reminiscent of a woodwind instrument.
Now it turns out there’s some basic math behind the bird’s songs, according to a study published today in the Proceedings of the National Academy of Sciences (PNAS). Hermit thrushes appear to prefer singing in harmonic series—a fundamental component in human music. The birds' musical taste is likely a product of biology, which might provide clues in an age-old debate: How much of human music’s origins are biological versus cultural?
Human songs are rooted in math. A harmonic series includes a fundamental base note followed by notes that increase in audio frequency based on multiples of that note. Scientists have long been intrigued by the idea that birdsongs might have some of the same hallmarks of human music, but studies on whether birds prefer harmonies have produced conflicting results.
Over the years, some ornithologists have claimed that the hermit thrush sings all sorts of musical scales, including major, minor and pentatonic (a common five-note scale). “The idea that hermit thrushes sing scales—particularly pentatonic scales—seems to have captured the human imagination and has been repeated so often that many people assume it is true,” says Emily Doolittle, a composer at Cornish College of the Arts in Seattle. Doolittle and her colleague Tecumseh Fitch, a biologist at the University of Vienna, set out to refute these claims—which made their results all the more surprising.
Male hermit thrushes sing 6 to 10 types of songs, all beginning with a long whistle. Most are high-pitched and fast. Using recordings from the Borror Laboratory at Ohio State, Doolittle and Fitch began analyzing the pitches of 114 different song types from 14 male hermit thrushes. “Listening to the songs full speed, they are very attractive, but we didn’t have any inkling that we would hear the harmonic series in them,” says Doolittle. They only began to pick up harmonies once they slowed the songs down. “It jumped out at us,” recalls Doolittle.
With this example of a hermit thrush song, you can hear the song itself and then the same notes in a harmonic series:
To ensure they weren’t just hearing what sounded familiar to their own musical ears, the researchers teamed up with statisticians Bruno Ginras and Dominik Endres to analyze the birds’ pitch selection. For comparison, they ran the same analysis on songs played on a woodwind instrument called an alphorn.
For 71 of the songs, they could determine distinct, stable pitches for at least 10 notes in the song. Out of those, 54 songs—about 70 percent of them—followed harmonic intervals. Here's another example of a harmonic hermit thrush song:
And here’s the slow version:
Just 5 percent of the bird songs were linked to random pitch generation and thus classified as nonharmonic. Here’s a nonharmonic hermit thrush song:
And here’s the slow version:
While some songs don’t follow harmonic series, it’s clear that the hermit thrush, for whatever reason, has a preference for singing ones that do. The songs don’t come from the physics of the hermit thrush’s vocal tract itself, because the tract isn’t flexible enough to simply produce all these series. So the bird has to be doing something to select the notes. For the hermit thrush, harmonic accuracy might be one way for females to evaluate male songs during mating seasons. Or harmonic series might simply be easier to remember, as they are for humans.
Doolittle is careful to point out that these birds aren’t structuring their songs based on scales. That would imply a degree of musical theory in birds that we have no way of proving—at least for now. Observations of the white-throated sparrow (Zonotrichia albicollis) and the nightingale wren (Microcerculus philomela) suggest these birds do not use intervals found in human music. Yet other species, like tropical boubou strikes (Laniarius aethiopicus) and musical wrens (Cyphorhinus arada) appear to prefer them. Another recent study found that domestic chickens show a preference for consonant notes, as well.
“The harmonic series is a physical phenomenon, not a culturally specific construct, like any scale, so it makes sense that this could be found in songs of a variety of different species,” says Doolittle. So what would it mean for humans if we do share these musical proclivities with birds?
“If an aspect of music is found not only in humans, but also in a variety of non-human species, this would suggest that there may be something in our shared biology that predisposes us to find that aspect interesting, or attractive, or easy to sing,” says Doolittle. The thrush study reinforces the notion that human music is a product of both biology and culture—but perhaps there’s more biology at play than we thought.