Shorebirds outfox predators by migrating farther north

Every year, shorebirds flap thousands and thousands of miles to the Northern Hemisphere, then back to the south. It’s an exhausting round trip. Yet some sandpipers and plovers head deeper into the Arctic, tacking as many as 2,000 miles onto their journey.

Why they do it has long puzzled biologists.

“Why wouldn’t they go in the low Arctic instead of the high Arctic? Why would you go so far north? It just increases the risk of getting lost or getting cold,” said Allan Baker, senior curator of ornithology and head of the Department of Natural History at the Royal Ontario Museum in Toronto. “It doesn’t make sense.”

In this week’s issue of the journal Science, researchers report that the birds aren’t gluttons for punishment. They face fewer risks from predators at higher latitudes -- so going the extra distance pays off.

Until now, scientists thought the birds decided how and where to fly based on food scarcity, parasite pressures and the risk of being eaten during migration. But Canadian and French researchers decided to test a further theory -- that the birds were trying to save their nests from egg-raiding predators.

Using quail eggs, the scientists set up 1,555 artificial nests at seven sites up and down the latticework of islands in the Canadian Arctic. The task was fairly straightforward because a shorebird nest is generally just a depression scratched out of the sand with three or four eggs laid inside. The scientists then monitored the sites every few days to see how many eggs had been stolen.

They found that the eggs were far less likely to be devoured by Arctic foxes and other predators at higher latitudes. With each one-degree increase in latitude, they calculated, there was a 3.6% decrease in risk of an egg being snatched.

From the study’s lowest point at Akimiski Island (53rd parallel north) to the northernmost site at Ellesmere Island (82nd parallel north), the likelihood of a shorebird having its eggs eaten by hungry animals decreased by a whopping 65%.

Scientists aren’t sure why. But it may be because predators are more numerous in the more agreeable climes farther south, where it’s easier to live and breed, said study coauthor Laura McKinnon, a University of Quebec researcher.

It is unclear how a warming Arctic will affect the fate of the birds, said Baker, who was not involved in the study. “Global warming does benefit the birds initially, we hope, by reducing some of the deaths from freezing,” he said. But, he added, the number of predators might increase in those warming areas. Scientists have yet to quantify the risks of predation, cold and exhaustion relative to one another, he said.

“Living on the edge, that’s what these birds really do,” he said. “It’s a balance.”

amina.khan@latimes.com