Following Birds at the Ends of the Earth

Two radar studies reveal migratory patterns in geographical extremes

Tracking migrating birds is always a challenge. It becomes even tougher for species that fly at night, travel so high in the sky that they can't be spotted by eye, or cross particularly remote regions of the globe. In two separate recent studies, ornithologists have for the first time used radar to monitor migrants in truly extreme situations: across the Arctic Ocean between Siberia and Alaska, and across the parched Sahara Desert of western Africa. Their studies have revealed unexpected details about the impressive transcontinental migrations of birds.

Crossing the Bering Strait

In one study, a group of Swedish researchers placed a bird-monitoring radar system on an icebreaker vessel that steamed north through the Bering Strait and far into the Arctic Ocean during August migration (Alerstam et al., Proceedings of the Royal Society B 274: 2523—2530). The radar measured the elevation and direction of migrants moving east from Siberia toward North America, and west from North America toward Asia.

The most impressive movements consisted of hundreds of high-flying flocks heading east toward North America from the Siberian Arctic. The researchers estimate that millions of birds cross the high Arctic every fall from Eurasia to North America, before turning south to enter this continent's major migratory flyways.

Using radar from an icebreaker vessel in the Bering Strait, researchers estimated that millions of birds cross the high Arctic from Eurasia to North America each fall. Arctic migrants Pomarine Jaeger (left), Bluethroat (middle), and Red Phalarope (right). © Gerrit Vyn (3)

These transcontinental migrants, which traveled primarily in the daytime, flew far too high to be observed and identified visually. However, evidence suggests that they are mostly shorebirds—such as Red Phalaropes and Long-billed Dowitchers—along with some species of tern and skua. By flying east to the North American mainland before turning south, they reduce the distance they must fly over inhospitable seas. In fact, the route is similar to the "great-circle route" that transcontinental airliners follow across the Arctic.

The study detected far fewer westward-flying birds. Most westbound migrants flew at lower elevations, migrated at night, and had radar signatures characteristic of songbirds. They probably were species like the Arctic Warbler, Bluethroat, and Northern Wheatear—birds that have colonized Western Alaska from Siberia, but still return to their ancestral Eurasian wintering grounds.

These avian journeys across the Bering region highlight the connectedness of the world's migratory birds. Some of the dowitchers wintering in San Francisco Bay probably breed in northern Russia, and some of the Arctic Warblers that hatch in Alaska's Denali National Park may fly all the way to Malaysia.

Crossing the Sahara Desert

Working in an utterly different environment, Swiss researchers set up a similar radar system deep in the central Sahara Desert during the southbound fall migration (Schmaljohann et al., Proceedings of the Royal Society B 274: 735?739). For a songbird, the trip north or south across the world's largest desert takes about 40 hours of flight time. Such a long flight over a region that offers no food or water for refueling raises an intriguing question: do the songbirds that cross the Sahara annually make the trip in one long flight, or do they land to rest during the heat of the day?

This question has been hard to answer directly. The Sahara is so vast that even if all of the roughly 4 billion birds stopped over, their average density would be only a few individuals per square mile. That makes them very difficult to locate and track. Further complicating the picture, studies of how migrants deploy their fat and water reserves offer conflicting guesses. Some suggest that migrants would fare best by flying nonstop, and others suggest that resting during the day is better, even though it prolongs the waterless stage of the journey.

In more hospitable environments such as those north and south of the Sahara, most songbirds migrate only at night. Using this information, paired with their radar locations far inside the central desert, the researchers predicted the time of day when a wave of songbirds should pass overhead, assuming the birds started their flights at sundown from the edge of the desert and flew nonstop.

Instead of the expected pulse of songbirds, the radar detected virtually no evidence of daytime migration over the Sahara. This absence provided strong evidence that most songbirds extend their migration time by resting during the day. During the northbound spring migration toward breeding grounds, the results were similar—except that about 20 percent of northbound birds did continue through the day, provided that there was a strong tailwind to help push them along.

The Sahara is one of the world's most dramatic migratory barriers, and many birds crossing it are probably pushed to the limits of their physiological capabilities. Because of the steady expansion of the desert's margins over the last few thousand years, the migration has undoubtedly become ever more difficult for the songbirds that must spend the heat of the day resting in its vast, arid landscape.

—Irby Lovette, director of the Lab's Fuller Evolutionary Biology Program

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