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Science

These remarkable frigate birds can fly without landing for months at a time

Frigate birds can fly without landing for up to two months at a time
A juvenile frigatebird.
(Henri Weimerskirch / CEBC CNRS)

On a tiny island off the coast of Mozambique, a young frigate bird emerges from his nesting place, looks north and flies off. And for 185 days, he keeps going ... for 34,000 miles.

Keep in mind that a complete trip around the equator is less than 25,000 miles.

The great frigate bird is the Energizer Bunny of migratory birds. Named after a kind of fast warship, frigates (Fregata minor) travel an average of 255 miles daily for days or months on end, often with little to no rest. Juvenile birds make even longer journeys.

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An excerpt from the film “Independence Days: In the Tracks of Young Marine Predators” describes recent research on how frigate birds manage to fly for months. (Aurelien Prudor / Henri Weimerskirch CEBC CNRS)

Even Christopher Columbus, as he sailed to the Americas in 1492, was impressed by the creatures, noting that the bird “does not alight on the sea nor depart from land 20 leagues” (70 miles) — which is, to be sure, an underestimate.

For seafaring birds, frigates are unusual. Their feathers aren’t waterproof and they have short legs, so even though they feed exclusively at sea, they can’t land on the water to get a meal of fish or squid. Instead, they just stay up there.

But how can a bird — even one with a 7.5-foot wingspan — stay aloft for so long?

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To maintain their marathon flights, the birds must learn a way to stay airborne while keeping the wing-flapping to a minimum, according to a new study in Science. Over the Indian Ocean, frigates rely on a set of atmospheric conditions to ride air currents like a roller coaster and propel them through their months-long journeys.

Between 2011 and 2015, zoologist Henri Weimerskirch, of the Chizé Center for Biological Studies in France, and colleagues attached solar-powered transmitters to about 50 frigates on Europa Island, which is in the channel between Mozambique and Madagascar. That allowed them to track the birds’ behaviors and movements after they left their breeding site.

From June to October, adult birds left the island and headed northeast to the Seychelles, some flying in huge loops around the equator and Indian Ocean. They traveled up to 48 days continuously, according to the data from the transmitters.

Young frigates departed separately from the adults and flew an average of 280 miles per day for more than two months on end. Some of them took short 8- to 48-hour breaks on islets along the way.

The transmitters revealed that the frigates follow an air current circulating around the edge of a windless basin in the Indian Ocean, called the doldrums. This belt, a reliable presence made up of trade winds that blow toward the equator and rising columns of warm air, called thermals, provides the birds with a sort of highway to follow.

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To gain altitude with a minimum of huffing and puffing, frigates get a lift from big pillowy cumulus clouds. Under the clouds, they make circular movements to soar upward on rising columns of warm air, reaching altitudes of 5,249 feet without flapping their wings.

Once they reach their preferred cruising altitude between 100 and 6,500 feet, they can glide for almost 40 miles until they need another lift.

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For a super boost, sometimes the birds will fly inside a cloud, where the upward drafts lift them 13 to 16 feet per second. Braving freezing temperatures and low oxygen, this gives the birds an extra bit of altitude to glide from when clouds are sparse.

If necessary, frigates will flap their wings to avoid landing on the water.

A male frigate bird displays his red throat pouch as a female bird flies above him.
A male frigate bird displays his red throat pouch as a female bird flies above him.
(Aurelien Prudor / CEBC CNRS )

With few pit stops on the birds’ ultra-marathon flights, how do they manage to eat? Do they ever sleep?

Since frigates spend very little time on land, it’s possible they sleep while airborne. But the study authors aren’t really sure when the birds would have time, since they’re completely motionless for only about two to 12 minutes at a time. It’s possible, the authors wrote, that animals such as frigates have evolved the ability to do without sleep when it’s more important they stay awake.

Eating is less of a challenge.

Columbus, ever the fan of the frigates, observed that the birds would show up when the ocean was “thick with tuna.” That’s no lucky coincidence, according to University of Washington biologist Raymond B. Huey and oceanographer Curtis Deutsch.

While flying at great heights, the frigates can easily spot schools of tuna or pods of dolphins foraging near the surface, the two wrote in an essay that accompanies the study. Those big predatory critters chase smaller fish and squid to the surface, where the frigates can nab them for a meal.

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Frigates can also catch flying fish as they jump out of the water. They’re even known to steal other seabirds’ catches.

According to Huey and Deutsch, the atmospheric conditions above the Indian Ocean sync up just right to provide frigate birds with transportation and meals.

Shifting trade winds in the west cause phytoplankton to blossom and support more fish and squid, which attract the tuna on their own migration.

But the scientists suggest climate change could disrupt this balance, and thus the frigates’ already perilous migration. Warming sea surface temperatures in the area are causing phytoplankton to decline, and climate forecasts also predict more intense tropical storms, such as cyclones.

“More variable atmospheric conditions in the future may become too challenging for a species that already seems to encounter extreme conditions during its lifetime movements,” the authors wrote.

sean.greene@latimes.com

Follow me on Twitter @seangreene89 and “like” Los Angeles Times Science on Facebook.

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