Europeans named the birds "frigate" for their resemblance to the swift warships. They look powerful, with their forked tails and long, slender wings; the shadows they cast on the sea surface could just as easily belong to a flock of pterodactyls. They make incredible feats of endurance — flying for two months without once setting foot on land, for instance — seem almost effortless: They ascend to altitudes of 2,000 feet and drop back down to sea level again with just a single flap of their wings.
"It is impressive," said Henri Weimerskirch, an ornithologist at the National Center for Scientific Research (CNRS) in France. "There is no other bird species like them."
Weimerskirch would know — he spent two years tracking 80 frigatebirds to understand their behavior, which seems to defy all laws of gravity and biology. Though they feed exclusively on fish, the creatures cannot swim. Instead they fly for weeks, even months, across the oceans of the tropics in search of food that swims close enough to the surface for them to snatch it out of the water.
They're able to do this, Weimerskirch reports Thursday in the journal Science, because of an incredible, intuitive mastery of air currents. Combining data from GPS trackers, heart rate monitors and accelerometers affixed to the birds with local weather reports, he and his colleagues figured out how frigatebirds take advantage of thermals and trade winds to fly over vast distances for long periods of time — all without expending much more energy than they would sitting in a nest.
Frigatebirds are the only seabirds that lack waterproof feathers — if they dive into the ocean or even land on its surface, the water will soak their plumage and prevent them from taking flight again. This should be a death knell to a species that dines exclusively on fish, but it's not. Frigatebirds can soar high above the ocean, eyes peeled for movement in the water below — a sign that a large predator, like tuna or dolphin, is scaring fish to the surface.
"When it sees its prey, it flies down head foremost to the water's edge, very swiftly takes its prey out of the sea with his bill, and immediately mounts again as swiftly; never touching the water with his bill," a 17th century observer wrote.
"It's a very, very specific niche and way of attacking prey," said Weimerskirch. "But the prey are very patchy ... it takes days and days to encounter one opportunity. So they have to cover very long distances to find it."
That's where the birds' soaring abilities come in. Their large, lightweight wings (their wing surface area to body mass ratio is higher than any other bird's) allow them to take advantage of the slightest breezes. They soar upward on thermals, much the way hang gliders do, and then glide back down gradually over the course of many miles. Unlike gliders, they also take advantage of the air currents within clouds to rise even higher when they need to; inside a cloud, they can ascend as fast as four to five meters per second. Few other birds and even most planes wouldn't attempt this because of the potential for turbulence.
The birds also know to avoid the "doldrums," the equatorial regions of the Atlantic and Pacific where light, capricious winds, unpredictable calms and sudden storms could spell doom for ancient mariners and modern birds.
Allowing air currents to do the work of lifting them makes frigatebirds' flight hugely efficient. The heart rate monitors Weimerskirch and his colleagues attached to the birds showed that they didn't flap their wings at all while gaining altitude — the energy costs of ascending thousands of feet in the air were the same as when they were sitting still.
And that, in turn, allowed them to travel vast distances over unimaginable periods of time. An adult bird can fly continuously for more than a month, covering 250 miles per day, without stopping once. Some crossed the Indian Ocean several times without setting a single foot on land.
It's still not clear how the birds sleep during these epic journeys. Weimerskirch said there's evidence that they can shut down half their brains as they ascend on updrafts.
"They are probably getting small bouts of sleep at this time — maybe two to three minutes," he said. A follow-up study with miniature loggers that monitor sleep will test this hypothesis.
Juvenile birds were especially strong endurance fliers — likely because they are going through a "dispersal phase," in which they journey as far as possible from their home nests, like 18-year-olds who insist they must fly to the other side of the country to go to college. One juvenile stayed aloft for more than two months. Another flew nearly 35,000 miles (a distance greater than the circumference of the Earth) in 185 days. On only four of those days did it land on an island to rest, and even then, only for a few hours.
But these birds owe a lot to their parents, who spend more time teaching their young the ways of the world than any other bird. Young frigatebirds spend six to eight months flying with their parents before setting out on their own.
"That's a very long period," said Weimerskirch. "And it's probably because it's complicated to learn how to fly. It takes time to learn to deal with all the conditions and all the constraints they have."
"When you see the flight pattern of a frigatebird," he added, "you understand."
But scientists are worried about how climate change might affect frigate birds' flight; shifting weather patterns could disrupt the flight techniques that frigatebirds spend so long learning. Warming oceans will likely become less productive, and the infrequent appearances of food that frigatebirds depend on may disappear.
"For either or both reasons," write ecologists Raymond Huey and Curtis Deutsch in an analysis of the study for Science, "great frigatebirds and other marine birds may no longer soar and glide over these vast regions."