Geoscience Reference
In-Depth Information
Seabirds, like these black-legged kittiwakes, choose precarious nesting sites that are inaccessible to terrestrial predat-
ors.
Seabirds are perhaps the most adaptable group of creatures on Earth, able to live on land, in the air, and in
the water. They have adapted to all the major environments on Earth, from the polar regions to the tropics.
They are distinguished from wading birds, like herons and egrets, and from shorebirds, by their ability to
thrive in the marine environment, often far from shore and for long periods of time. To do so, they have had to
evolve a means of ridding themselves of salt through salt glands, located in the eye orbit, where they act as
extra-renal kidneys. In some cases, seabirds spend months, even years, at sea, only coming to shore to breed.
Overall, however, seabirds have several characteristics in common: they breed in vast colonies, are mono-
gamous, and forage over wide distances—all behaviors that have evolved to increase their chances of survival.
But seabirds display a number of dramatic differences from passerines, or land birds, in their life history. They
are much longer-lived, with life spans of twelve to sixty years compared with five to fifteen for land birds.
They begin to breed at a later date (two to nine years compared to one to two), they produce smaller clutch
sizes—often one egg only—and their chicks are slow to mature.
Why seabirds are so different in these aspects of their lives is not well understood. It has been hypothesized
that seabirds' small clutch size and the slow growth of the chicks may be related to limited food supply near
colonies or even, in some cases, to the depletion of food supply by the foraging seabirds themselves. There is
little firm evidence to support this “energy-limitation hypothesis,” however. The amount of food that adults
