Songbirds’ epic migration routes

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A Swainson’s thrush wearing a geolocator.
Credit: Kira Delmore / University of British Columbia

The route a tiny songbird takes on its annual migration from Canada to Central and South America is written in its genes, a new study shows.

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Swainson’s thrush, snapped here in British Columbia, Canada, migrate up and down the length of the Americas.
Credit: Darren Irwin / University of British Columbia

An international team of biologists tracked the routes of two subspecies of the migratory Swainson’s thrush (Catharus ustulatus) and analysed differences in their genome.

They found a single cluster of around 60 genes – which includes circadian rhythm genes – govern the path they take.

The work was published in Current Biology.

The most common migrant bird in North America, the Swainson’s thrush makes its return trip from Canada to Mexico, and as far as Argentina, each year.

But there are subspecies – a coastal variety with reddish-brown back feathers and an inland, which has olive back feathers.

They also take different migratory routes: the coastal birds track closely to the west coast of the US while the inland animals cut straight across to Florida, where they then fan out down to Central and South America.

What’s behind these different routes?

Kira Delmore, then at the University of British Columbia in Canada and now at the Max Planck Institute for Evolutionary Biology in Germany, and colleagues tracked the little songbirds of both subspecies as well as hybrids using coin-sized geolocators, strapped to the birds’ back.

They also sequenced bird genomes to determine differences between the groups.

While the subspecies are related – they interbreed in mountains northeast of Vancouver – just one package of around 60 genes on chromosome 4 determined the route a bird took when it flew the coop.

It’s not the first time migratory habits have been found to have a genetic basis – a 2009 study showed migratory monarch butterflies had different genes to their resident counterparts.

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