How a bird forgot its mother tongue
New research to be published next month reports that this European bird gradually lost the syntactical structure of its song over the last million years, as it colonized island after island.
Robert Lachlan/Duke University
Each time the chaffinch arrives on a new island, it forgets its speciesâ€™ traditional song a little more.
In the first instance of quantifying how bird songs change over time, new research reports that this European bird gradually lost the original syntactical structure of its song over the last million years, as it colonized island after island. Now, just the chaffinches in the birdâ€™s ancestral homeland, mainland Europe, are fluent in the song, while the birds out on the last colonization stop, in the Canary Islands, sing an un-patterned version.
That the chaffinchâ€™s loss of syntax followed such a neat, directional pattern suggests that the evolution of the chaffinchâ€™s song is not random at all, but somehow yoked to the birdâ€™s island hopping. That find also furnishes a new question: What about moving to a new island pressures a bird into unraveling its song? Answer that question, the authors say, and it might be possible to better clue into the process through which one species of bird becomes two species, and then four, and so on.
â€śWe found this really striking pattern of evolution,â€ť says Robert Lachlan, a postdoctoral fellow at Duke University and the lead author on the paper, to be published in October in the journal Current Biology.
â€śSomething that happened during or shortly after the process of colonization was responsible for the change in song structure,â€ť he said. â€śBut we canâ€™t say for sure what it is yet.â€ť
The chaffinch, or, Fringilla coelebs, is one of the most common birds in Europe, but it is not a homely one. Blue-capped, red-breasted, and black-white feathered, the petite male bird looks as if it was painted using a paint-by-number set â€“ except for the Hamburgler-like dark band across its eyes. The female bird is duller in color than her male counterpart, as is common in most species in which the female is burdened with chick rearing and so must be choosy about which male is good enough, or colorful enough, to father her young.
The chaffinchâ€™s song, often transliterated as â€śsusk-WEET,â€ť is a species-unique tune. Just male chaffinches sing it, calling to their Juliets to come to the balcony, or the bush, or the park bench, or wherever. The bantam Juliets then recognize the singing as from one of their own speciesâ€™ Romeos, instead of one of the dozens of other species tweeting garbled prose.
â€śItâ€™s a long-distance mating signal,â€ť says Dr. Lachlan. â€śThe song is often the first point of contact for a female looking for her mate and her mate.â€ť
For years, sensitive listeners had suspected that chaffinches on the European mainland and chaffinches out on the islands did not sing their melodies quite alike. But just what that difference in song was had been unknown, says Lachlan. Unknown, too, was if this seemed bird accent was happenstance, a chance aberration in the birdâ€™s song each time they arrived at a new island. Or, was there a pattern to how the birds appended their song each time they relocated?
What if something more than chance was driving the changes in the birdsâ€™ song?
Beginning what was to be 15 years of research, Lachlan recorded the songs of 723 male birds in 12 different populations from the European mainland, the Azores, and Canary Islands. He then used an algorithm to divide the birdâ€™s song into syllables. Next, he used another algorithm to assess the randomness of each birdâ€™s song: After the bird sang syllable â€śA,â€ť for example, how predictable was the next syllable?
In mainland Europe, the chaffinches all sang the same, predictable song, Lachlan found. But as Lachlan traced the birds over their some million-year migration out to the islands, he found that, on each stop along the way, the song became less and less ordered. On the furthest island out in the birdsâ€™ long migration, Gran Canaria in the Canary Islands, the birds still sang the same syllables, but without a discernible pattern.
â€śIn each step of the colonization chain, they lost a little bit of their syntactical structure,â€ť says Lachlan.
That find upended conventional assumptions about how a birdâ€™s song would develop once it landed in a new place. Since each stop is an independent event, it had been thought that changes in a birdâ€™s song would be random, with the song becoming more or less ordered depending on chance occurrences.
Not so, according to Lachlanâ€™s research.
â€śItâ€™s not random drift,â€ť says Lachlan. â€śIt seems to be a directional process, which suggests thereâ€™s some force underlying it.â€ť
But what that force is, Lachlan says, is still mysterious.
Thatâ€™s because the factors controlling for song learning are still unclear, he said, noting that researchers have not yet worked out how the odd two-step of genetics and culture produces changes in bird behavior.
Biologists have known for decades that bird song is in part genetically encoded. In 1954, British zoologist William Homan Thorpe reared a group of chaffinchÂ chicksÂ in isolation from their species. When the birds were still young, he played them recordings of tweets of their own species, as well of those of other species. Which songs, he wanted to know, would these chaffinches learn?
Well, in large part the chaffinches sung back the â€śsusk-WEETâ€ť of their own kind, suggesting that there must be some genetic limits on which songs a chaffinch will and won't learn.
â€śBirds have a bias in what songs theyâ€™ll learn,â€ť says Lachlan. â€śThey wonâ€™t learn just anything they hear.â€ť
So, if song learning is genetic, the chaffinchâ€™s loss of syntax might also have genetic underpinnings, says Lachlan. Letâ€™s say that a male bird has an unusual gene that allows it to sing a large repertoire of songs. Over time, this gene will be favored in natural selection over genes that restrict a birdâ€™s songbook. Thatâ€™s because the female bird that can recognize this maleâ€™s song will have more males from which to choose and will have better odds of mating, says Lachlan.
Still, genetics donâ€™t quite tell the whole song of how the chaffinch gets its song. Song learning is not just genetic â€“ itâ€™s also cultural.
When birds first pop off to a new island, their population size is small, says Lachlan. This lowers the odds that a young bird is exposed to adults singing the chaffinchâ€™s traditional song. Without that template, the little bird will improvise, singing syllables without a pattern, a problem familiar to anyone who has ever tried to stir up a grandparentâ€™s recipes from the old country, with dubious success.
â€śDuring and after island colonization, cultural traditions and cultural evolution are disrupted,â€ť says Lachlan.
Teasing out where changes in song are controlled for in culture, and where in genetics, is the next research challenge, he said.