Among
the species of seabirds that use the windswept cliffs of the Atlantic coast
of
Canada
in the summer to mate, lay eggs, and rear their young are common murres,
Atlantic
puffins, black-legged kittiwakes, and northern gannets. Of all the birds on
these
cliffs, the black-legged kittiwake gull is the best suited for nesting on
narrow
(5)
ledges. Although its nesting habits are similar to those of gulls that nest
on flat ground,
there
are a number of important differences related to the cliff-nesting habit.
The
advantage of nesting on cliffs is the immunity it gives from foxes, which
cannot
scale the sheer rocks, and from ravens and other species of gulls, which
have
difficulty
in landing on narrow ledges to steal eggs. This immunity has been followed
(10)
by a relaxation of the defenses, and kittiwakes do not react to predators
nearly as
fiercely
as do ground-nesting gulls. A colony of Bonaparte's gulls responds to the
appearance
of a predatory herring gull by flying up as a group with a clamor of alarm
calls,
followed by concerted mobbing, but kittiwakes dimply ignore herring gulls,
since
they
pose little threat to nests on cliffs. Neither do kittiwakes attempt to
conceal their
(15)
nest. Most gulls keep the nest area clear of droppings, and remove empty
eggshells
after
the chicks have hatched, so that the location of the nest is not given
away.
Kittiwakes
defeacate over the edge of the nest, which keeps it clean, but this
practice, as
well
as their tendency to leave the nest littered with eggshells, makes its
location very
conspicuous.
(20)
On the other hand, nesting on a narrow ledge has its own peculiar problems,
and
kittiwake
behavior has become adapted to overcome them. The female kittiwake sits
when
mating, whereas other gulls stand, so the pair will not overbalance and
fall off the
ledge.
The nest is a deep cup, made of mud or seaweed, to hold the eggs safely,
compared
with the shallow scrape of other gulls, and the chicks are remarkably
(25)
immobile until fully grown. They do not run from their nests when
approached, and if they should come near to the cliff edge, they
instinctively turn back.
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