Frigatebirds ride updrafts, stay aloft for weeks without flapping

6/30/2016 | 0

Female Great Frigatebird in flight, in Europa Island, photo by Aurelien Prudor CEBC CNRS.

Female Great Frigatebird in flight, in Europa Island, photo by Aurelien Prudor CEBC CNRS.

Beautiful, long-winged Magnificent Frigatebird was the subject of a memorable article in our December 2013 issue by writer and tour leader Mark Hedden.

The large, mostly black seabird is one of the world’s five species of frigatebird and the only one to breed in the tropical Atlantic. And it’s a species of mounting concern, since its breeding colonies in the Caribbean have been disappearing.

Using solar-powered satellite-tracking devices, Hedden wrote, biologists from the U.S. Fish and Wildlife Service and the Gainesville-based Avian Research and Conservation Institute showed that the birds wander much more widely than imagined. A female tracked between October 2012 and August 2013, for example, roamed over the open waters of the Caribbean, effortlessly touching the shores of Florida, Cuba, and the Yucatán Peninsula, seemingly without a care in the world.

That’s a remarkable accomplishment. Frigatebirds have long, narrow, pointed wings and a long, narrow, deeply forked tail, and they have the highest ratio of wing area to body mass of any bird. They are also unusual in that their feathers are not waterproof and their legs are small, so they avoid landing on the sea surface.

H. Weimerskirch with a juvenile Great Frigatebird, photo by Henri Weimerskirch CEBC CNRS.

H. Weimerskirch with a juvenile Great Frigatebird, photo by Henri Weimerskirch CEBC CNRS.

How frigatebirds stay aloft

So how do they manage to stay aloft for so long? According to a just-published study in the journal Science, researchers working halfway around the world appear to have the answer.

Stationed on Europa, a tiny atoll between Mozambique, in southeastern Africa, and the island of Madagascar, researcher Henri Weimerskirch and colleagues outfitted dozens of Great Frigatebirds, a sibling of Magnificent Frigatebird, with solar-powered Argos transmitters, data-loggers, and GPS accelerometers and then tracked the birds’ movements in the Indian Ocean after the breeding season.

The data they gathered revealed that adults stayed continuously on the wing for as long as 48 days. Juveniles made nonstop flights lasting up to 2.1 months.

The birds cover substantial distances — on average, 410 kilometers (255 miles) each day. Remarkably, juveniles were found to embark on even longer journeys than adults, and to do so without adult companions, suggesting that their flight patterns reflect a genetically encoded behavior.

Most of the Great Frigatebirds traveled at an altitude that required little flapping to stay aloft, somewhere between 30 and 2,000 meters (98 and 6,562 feet). Only when foraging, which is much more energy-intensive, did they drop down to elevations beneath 30 meters.

Hitching a ride upward

While aloft, write Weimerskirch and his colleagues, the birds used the circular movements of upward drafts under cumulus clouds to soar to higher altitudes. Lifted ever upward by the drafts, the frigatebirds ascended as high as 1,600 meters (5,249 feet) without flapping, sometimes rising at rates of 4-5 meters (13-16 feet) per second.

Once at a higher altitude, the birds could then glide downward for long distances, flying with side winds to achieve the highest ground speeds, before hitching a ride on the next upward draft. Weimerskirch and his colleagues characterized the resulting movement as a “complex zig-zagging, roller-coaster movement.”

According to the study data, the birds did not beat their wings or beat them only rarely during more than 80 percent of the time they were in flight at sea. Just as remarkable, the birds’ heart rates occasionally attained values as low as when the birds were resting on their nests.

Were the frigatebirds ever asleep?

“Animals such as frigatebirds may have evolved the ability to dispense with sleep when ecological demands favor wakefulness such as during extended flights,” write Weimerskirch and his team, “but studies are needed to determine how they sleep during much longer-lasting flights.”

Read the abstract

Henri Weimerskirch, Charles Bishop, Tiphaine Jeanniard-du-Dot, Aurélien Prudor, and Gottfried Sachs (2016) Frigate Birds Track Atmospheric Conditions over Months-long Transoceanic Flights, Science, Vol. 353, No. 6294 (1 July 2016): pp. 74-78. Abstract.

Why researchers are in a hurry to learn the many secrets of the Magnificent Frigatebird (December 2013 issue).

New research uncovers the wanderings of Magnificent Frigatebirds (Nov. 12, 2013).

Seabird populations declined 70 percent throughout the modern industrial era (Aug. 20, 2015).

See photos of frigatebirds.

 

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