The song of the hermit thrush sounds a bit like an orchestra warming up: the experimental trill of a flautist, a violinist testing his vibrato on a few high notes, all just beginning to suggest a larger melody.
This undulating song can be heard throughout North America in the summer. But moving east to west, a keen listener might notice that the bird’s repertoire changes: lower notes introduce each song, like a viola has been added to the mix, and the trills that follow are shorter.For biologists, this change is the anthem of evolution.
One million years ago, the hermit thrush population was separated, likely by a large glacier that cleaved North America in half. A recent study shows that this separation has had an audible effect: today, the eastern and western hermit thrush populations sing their songs differently.
As in many songbirds, male hermit thrush are the singers of the species, learning their songs as chicks from other adult males—fathers, uncles, brothers—around their nest. But that learning is imperfect, and every new singer introduces small differences to the song, the way changes appear in a passed phrase during a game of Telephone.
With the eastern and western populations separated, those differences couldn’t be shared, explains lead author Sean Roach. The hermit thrush songs that we hear today have diverged—as though they have passed through two concurrent and very long games of Telephone.
“If there are sufficiently large song differences between populations, it can get to the point where they don’t recognize what they’re hearing as being from the same species,” Roach says.
Above, the song of a western-type hermit thrush, recorded in Baja California. Listen for longer introductory notes in a more limited frequency, with shorter phrases post-introduction.
Over time, the accumulation of differences can create a feedback loop: as two groups of a species begin to sound more and more unlike, they are less likely to recognize each other as potential mates. This makes the groups less likely to share both genes and songs, pushing their songs to become even more dissimilar.
In this way, sounds can be both a signal and a driver of evolution, making vocalizations rich territory for biologists looking to spot evolution in action.
On the islands of Hawaii, roughly four new species appear in the cricket genus Laupala every million years—an explosion, by evolutionary standards, and the highest documented for arthropods, which usually see new species more on the order of 0.16 per million years. The driving force behind this evolutionary outbreak? Males in different species sing their pulsing courtship song at a different rate, so researchers theorize that the songs have something to do with it—that over time, female preference for different songs has pushed the group to continually diverge.
Differences in sounds can also help species to share without out-competing each other. Like many family members, five bat species from the European genus Myotis share a lot—they look physically similar, live in the same areas, and have similar strategies for hunting. Yet each species of bat has a slightly different echolocation signal, allowing each species to target a slightly different prey.
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