Natures wonders: frigatebird flight (Introduction)

by David Turell @, Friday, July 01, 2016, 02:32 (1305 days ago) @ David Turell

They can stay aloft for as long as 63 days gliding on updrafts:

"Frigatebirds are really strange in many aspects of their life history,” says Henri Weimerskirch at the Centre for Biological Studies, Chizé, France. Unlike other long-distance travelling seabirds like albatrosses, frigatebirds' feathers lack waterproof oil so they can't take a break on the sea. Instead, they have to save energy by coasting for kilometres while minimising wingbeats.


"The devices revealed globetrotting flights that lasted up to 63 days without a rest. Only alpine swifts can fly for longer. Wandering juveniles travelled the farthest, with one chalking up 55,000 km in 185 days with only four days of rest on islands.

"The frigatebird's migratory behaviour is unique among birds: while most birds avoid clouds because of their turbulence, frigatebirds seem to seek them out. “These frigatebirds do it intentionally,” Weimerskirch says. The birds ride on the strong updrafts under cumulus clouds in the open ocean to gain altitude.


"They usually climb to the base of the cloud layer, about 700 metres up, before entering a long descending glide. But when the next cloud is far away, they keep ascending into the layer, up to 4000 metres - above the height where water droplets begin to freeze.

"This lets them coast for more than 60 kilometres until they find another updraft. “This was a very surprising result,” Weimerskirch says.

"The birds drift with the trade winds on their ascent, and descend with the prevailing breeze to pick up speed. The scientists call the resulting zigzag a “roller coaster” flight pattern. In total, the frigatebirds climbed an average of 15.4 kilometres a day - that's more one and a half times the height of Mount Everest.

"Since frigatebirds can't take a break in the water, they have to snooze on the wing. The birds stopped flapping while rising in updrafts, so that's probably when they sleep, says Curtis Deutsch at the University of Washington. “When they're gliding, they need to be scouting out opportunities,” he says.

"The migratory paths track the wind belt around the doldrums or equatorial calms at the centre of the Indian Ocean. This is a good way to look for prey, Deutsch says. Predatory fish like tuna also skirt the edge of the doldrums. The predators chase smaller creatures up to the surface, where frigatebirds can snatch them."

Comment: They use the same techniques as human glider pilots.

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