Birds fly hundreds of kilometres every night when they migrate south for winter. Now it seems all North American land birds use one of just three routes when they seek out the sun.
Different bird species often converge on the most efficient routes, resulting in clustered routes called "flyways", not that dissimilar to a human highway. The flyways of water birds are well-established, but land birds migrate at night and less is known about the paths they follow.
So Frank La Sorte and his colleagues from Cornell University in Ithaca, New York, mapped the flyways of 93 land birds, from the 2-gram ruby-throated hummingbird to the 500-gram broad-winged hawk. All fly at night and roost and feed during the day.
My way to the flyway
Using a database called eBird that stores reports from birdwatchers, the team created maps showing spring and autumn migration routes. Three flyways emerged: an eastern route used by 45 species, a central one with 17 and a western one with 31.
"This is one of the most detailed analyses of flyways," says Samantha Franks from the British Trust for Ornithology in Thetford, UK.
The three flyways seem to be a hangover from the ice age, which began 2.6 million years ago and periodically covered most of North America in ice. "The glaciers extended into the centre of the continent," says La Sorte, "isolating the eastern and western portions." Later the central portion became the Great Plains.
That makes sense, says John Faaborg from the University of Missouri in Columbia. "Given that the eastern half of the US is forested, the west is mountainous, and grasslands split those two things up, it is not surprising that eastern forest birds stayed east and western forest birds stayed west and grassland birds went up the middle."
Riding the winds
The birds take different routes in spring and autumn, especially in the eastern and western flyways. In spring, as they head north, they seem to ride the winds to speed up their journey, aligning their routes with a night-time jet stream that flows from the Gulf of Mexico into the Great Plains. The route is longer, but they enjoy stronger tailwinds.
La Sorte says climate change will probably strengthen the jet stream. "We expect the costs of migration to potentially change, becoming energetically less expensive in the spring and more in the autumn."
Such shifts in wind strength do matter, according to a second study. Over a nine-year period, strong westerly winds during spring migration made yellow warblers less likely to survive the year (PLoS ONE, DOI: 10.1371/journal.pone.0097152).
Journal reference: Journal of Biogeography, DOI: 10.1111/jbi.12328
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