http://www.nytimes.com/2003/01/28/science/28OBSE.html
January 28, 2003
Tracking High Fliers
By HENRY FOUNTAIN
The magnificent frigatebird is a fine example of a seabird, foraging
over the water for food to return to its coastal nests.
There's just one problem: the magnificent frigatebird can't land on
the sea. Its wings are so ungainly, spanning about seven feet, that if
it landed on water, it wouldn't be able to take off again. So the bird
is forced to spend long hours aloft.
Just how long — and how high — the birds fly has now been documented.
Henri Weimerskirch and colleagues from the National Center for
Scientific Research in Villiers en Bois, France, attached altimeters
and satellite transmitters to magnificent frigatebirds in French
Guiana, and recorded data during 42 foraging trips.
The researchers found that the birds remained in the air night and
day, with the shortest trip lasting eight hours and the longest almost
four days. They reached altitudes as high as 7,700 feet, although the
average was on the order of several hundred, and came close to the sea
only once every seven hours, on average, in search of food.
The birds fly the way gliders do — by catching pockets of rising warm
air, or thermals. Most glider pilots prefer soaring near mountains,
where thermals are strong; the frigatebird, being in the tropics, must
make do with relatively weak thermals that form beneath cumulus clouds.
Still, their huge wings and light weight (about 3 pounds) enable them
to soar for hours.
The birds mainly eat fish or other seafood that is forced to the
surface by underwater predators like tuna. As a result, dining
opportunities are few.
The researchers say the birds' large wingspan and ability to remain
aloft are no doubt evolutionary adaptations to the difficulties of
finding food
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