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.
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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 birds’ long migration, Gran Canaria in the Canary Islands, the birds still sang the same syllables, but without a discernible pattern.Skip to next paragraph
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“In each step of the colonization chain, they lost a little bit of their syntactical structure,” says Lachlan.
That find upended conventional assumptions about how a bird’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 bird’s song would be random, with the song becoming more or less ordered depending on chance occurrences.
Not so, according to Lachlan’s research.
“It’s not random drift,” says Lachlan. “It seems to be a directional process, which suggests there’s some force underlying it.”
But what that force is, Lachlan says, is still mysterious.
That’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 chaffinch chicks 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 “susk-WEET” of their own kind, suggesting that there must be some genetic limits on which songs a chaffinch will and won't learn.
“Birds have a bias in what songs they’ll learn,” says Lachlan. “They won’t learn just anything they hear.”
So, if song learning is genetic, the chaffinch’s loss of syntax might also have genetic underpinnings, says Lachlan. Let’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 bird’s songbook. That’s because the female bird that can recognize this male’s song will have more males from which to choose and will have better odds of mating, says Lachlan.
Still, genetics don’t quite tell the whole song of how the chaffinch gets its song. Song learning is not just genetic – it’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 chaffinch’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 grandparent’s recipes from the old country, with dubious success.
“During and after island colonization, cultural traditions and cultural evolution are disrupted,” says Lachlan.
Teasing out where changes in song are controlled for in culture, and where in genetics, is the next research challenge, he said.
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