Unladen barn swallow. Credit: Katsura Miyamoto (CC BY 2.0)
Unladen barn swallow. Credit: Katsura Miyamoto (CC BY 2.0)

On The Evolution of Migration

Every autumn, the swallow may fly south with the sun. It is joined by the house martin, the plover, and hundreds of other species of birds. After spending the summer in temperate breeding grounds, where both daylight and food are plentiful, they head south before both resources fade in the winter. When spring returns, so do these migrants.

Their treks can be epic. The bar-tailed godwit flies from Alaska to New Zealand. The Arctic tern makes a 70,000 kilometre round-trip from one pole to another. Even less ambitious migrations still involve small birds, the size of your hand, crossing whole continents.

Migration evolved from stagnation. The ancestors of these birds stayed in the same place all-year round, and gradually, they shifted either their breeding grounds or their wintering ones. Most scientists believe that the former happened: that tropical birds gradually moved their breeding grounds north, either to chase a glut of summer food or to leave their competitors behind. But some think that the opposite happened: birds in temperate climates gradually moved south to escape the harsh winters.

Fossils would normally help to settle these competing explanations, but small songbirds have tiny, hollow bones that don’t fossilise very well. So, Benjamin Winger and Richard Ree from the University of the Chicago used a different approach. They built a mathematical model that uses the ranges of modern birds and their evolutionary relationships to reconstruct the historical ranges of their ancestors. It’s a tool for looking at geography, back through history.

They focused on New World emberizoids—an unfamiliar name for a group of extremely familiar songbirds including warblers, sparrows, blackbirds, orioles and tanagers. “They’re really familiar birds to birdwatchers and ornithologists,” says Winger (providing a great example of nominative determinism). “If you go out during spring migration, these are many of the birds you’ll see.”

Most of the emberizoids live in the tropical parts of the Americas, and stay there throughout the year. But they also include 120 species of migrants, which travel north in the spring to breed. At first glance, it looks like these birds originated in the tropics and gradually expanded north, supporting the dominant idea. But Winger and Ree found that the opposite scenario was more common. Their model told them that emberizoid migrants were twice as likely to evolve out of North America as out of the tropics.

The team also found that for many species, migration was a gateway drug for a permanent tropical existence. From an all-year life in the north, they started moving south in the winter, until some of their descendants just stayed in the tropics all the time.  They cashed in their return tickets for one-way passes and never went back.

“This is something that people have observed at a smaller scale,” says Winger. “The barn swallow is a nice example. They breed in North America and winter in the tropics, but there’s a population that has begun breeding in Argentina.”

Of course, this only tells us about how some birds evolved their long migrations, but not why they did so. That’s a topic for another study (and another post; although here’s a good primer). But Winger says the how is important. “There are many competing hypotheses for the evolutionary drivers of migration, but they rely on certain patterns of geographic change.” By better understanding how those patterns changed, he hopes to bring more clarity about why they did so.

“In some ways, it’s intuitive,” he adds. “The winter is a pretty harsh place to be, so birds that leave, or evolve the ability to leave, will go on to survive.”

Reference: Winger, Barker & Ree. 2014. Temperate origins of long-distance seasonal migration in New World songbirds. PNAS. http://dx.doi.org/10.1073/pnas.1405000111

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