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For the first time, scientists have investigated the mysterious behavior
of the magnificent frigatebird (Fregata magnificens), a tropical
sea bird well-known among ornithologists, but which had not really been
studied until now. Although these birds spend their time flying over the
sea, foraging for food, they cannot land on water. Their feathers are
not waterproof and their feet are too short to push them out of the water.
Frigatebirds are nonetheless perfectly adapted to their environment, as
has been shown by Henri Weimerskirch and his team from the CNRS "Centre
d’études biologiques" (Center for biological studies)
in Chizé. They conducted a study at the birds' reproduction site,
located on the nature reserve of Grand Connétable in French Guiana.
In an article to appear in
the January 23 issue of the journal Nature, these researchers explain
how frigatebirds make use of thermals, which are ascending air currents,
to save as much energy as possible while in flight. Because their body
weight is very light in comparison to their wing surface, they can glide
in the air at altitudes of up to 2,500 meters. Then they glide downward,
taking advantage of the next current. This flight strategy, which limits
the bird's physical efforts, is the same as that used by migratory birds
during long flights over land. Migratory birds, however, avoid flying
over the sea due to a lack of thermals, while frigatebirds fly over the
sea. As it turns out, ascending air currents are not found over the sea
except in tropical regions where the waters are warm enough to create
such currents on a continuous basis. Frigatebirds can therefore fly in
such an environment, night and day, using this technique.
In order to investigate the movements of magnificent frigatebirds, the
researchers took measurements in three dimensions. In addition to fitting
the birds with Argos satellite transmitters, they also fitted them with
altimeters, which allowed the scientists to observe that the birds only
occasionally come close to the sea surface to catch their prey. They catch
flying fish or squid that are driven above the surface by underwater predators,
mainly schools of tuna or dolphins. In order to identify such feeding
opportunities, which are very rare, this technique requires long hours
of flight at high altitudes. Frigatebirds rarely feed their young, which
consequently grow very slowly. The species is, however, well-adapted:
it has a low reproductive rate and parent birds care for their young for
over one year, the longest period of parental care of any bird.
Researcher
contact:
Henri Weimerskirch
Tel: + 33 5 49 09 78 15
e-mail: henriw@cebc.cnrs.fr
Life Sciences Department contact:
Françoise Tristani
Tel: +33 1 44 96 40 26
e-mail: francoise.tristani@cnrs-dir.fr
Press contact :
Martine Hasler
Tel : +33 1 44 96 46 35
e-mail : martine.hasler@cnrs-dir.fr
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