http://www.npr.org/sections/thetwo-way/2016/06/30/484164544/non-stop-flight-how-the-frigatebird-can-soar-for-months-without-stopping

Frigatebirds, seagoing fliers with a 6-foot wingspan, can stay
aloft for weeks at a time, a new study has found. The results paint an
astonishing picture of the bird's life, much of which is spent soaring inside
the clouds.

Frigatebirds are unique among aquatic birds. Their feathers are not waterproof,
so they can't rest on the waves. Males sport a vivid red pouch along their
throats that they inflate when trying to attract females. They're known for
stealing food from other seabirds.

Since the frigatebird spends most of its life at sea, its habits outside of when
it breeds on land aren't well-known — until researchers started tracking them
around the Indian Ocean. What the researchers discovered is that the birds'
flying ability almost defies belief.

Ornithologist Henri Weimerskirch put satellite tags on a couple of dozen
frigatebirds, as well as instruments that measured body functions such as heart
rate. When the data started to come in, he could hardly believe how high the
birds flew.

"First, we found, 'Whoa, 1,500 meters. Wow. Excellent, fantastique,' " says
Weimerskirch, who is with the National Center for Scientific Research in Paris.
"And after 2,000, after 3,000, after 4,000 meters — OK, at this altitude they
are in freezing conditions, especially surprising for a tropical bird."

Four thousand meters is more than 12,000 feet, or as high as parts of the Rocky
Mountains. "There is no other bird flying so high relative to the sea surface,"
he says.

Weimerskirch says that kind of flying should take a huge amount of energy. But
the instruments monitoring the birds' heartbeats showed that the birds weren't
even working up a sweat. (They wouldn't, actually, since birds don't sweat, but
their heart rate wasn't going up.)

How did they do it? By flying into a cloud.

"It's the only bird that is known to intentionally enter into a cloud,"
Weimerskirch says. And not just any cloud — a fluffy, white cumulus cloud. Over
the ocean, these clouds tend to form in places where warm air rises from the sea
surface. The birds hitch a ride on the updraft, all the way up to the top of the
cloud.

Frigatebirds have to find ways to stay aloft because they can't land on the
water. Since their feathers aren't waterproof, the birds would drown in short
order. They feed by harassing other birds in flight until they regurgitate
whatever fish they've eaten and the frigatebird takes it. Or they fly over a
fish-feeding frenzy on the ocean surface and scoop up small fish that leap out
of the water to escape larger fish.

So in between meals, apparently, frigatebirds soar ... and soar ... and soar.

In one case, for two months — continuously aloft.

"Absolutely incredible," says Curtis Deutsch, an oceanographer at the University
of Washington. "They're doing it right through these cumulus clouds. You know,
if you've ever been on an airplane, flying through turbulence, you know it can
be a little bit nerve-wracking."

One of the tagged birds soared 40 miles without a wing-flap. Several covered
more than 300 miles a day on average, and flew continuously for weeks. They are
blessed with an unusual body. No bird has a higher ratio of wing surface area
compared with body weight — something called "wing loading."

Writing in the journal Science, the researchers discovered that frigatebirds
have also capitalized on a lucky coincidence. Winds that form these updrafts in
the atmosphere also disrupt waves at the sea surface.

"We found that there's a remarkably good correspondence between those two
things," Deutsch says. And when the regularity of waves is disrupted, deeper
water rises to the surface, carrying with it things such as phytoplankton that
attract small fish. The small fish attract bigger fish, which creates the
feeding frenzy that frigatebirds need to dine.

So it seems the life of a frigatebird is simply hopping off at the bottom of
this atmospheric roller coaster, eating and getting back on again to search for
the next meal.