Geoscience Reference
In-Depth Information
bring to the nest seems to be regulated by the chick's food-begging behavior. Adults respond to the chick's
needs rather than bringing the maximum amount of food to the nest. In addition, both parents are observed to
loaf at the nest during the rearing period, suggesting that the amount of food is not the limiting factor. Even if
the parents were to bring more food to the chicks, it is not known whether some physiological or genetic con-
straint would prevent them from assimilating it.
Almost all seabirds—some 95 percent—are colonial nesters, a much higher proportion than in other bird
families. (A colony can be loosely defined as a group of individuals at a breeding site.) Gulls, terns, and auks
are predominantly colonial. Although colonial living is not exclusive to seabirds, theirs is perhaps the ultimate
example of coping in close quarters. Living close together requires constant communication, which is probably
why large seabird colonies, sometimes numbering in the millions of pairs, are such noisy affairs, with waves of
sound constantly rippling through them.
One obvious advantage of colonial breeding is strength in numbers. Large colonies are better able to defend
themselves against predators. For seabirds, however, this advantage probably only applies to avian predators,
such as ravens, eagles, or even other seabirds, like the predatory gulls, since they are largely defenseless
against mammalian predators, such as cats, dogs, foxes, or even rats. (Extirpation of gannets took place in two
southern colonies, on Seal Island off Yarmouth and Gannet Rock in the Bay of Fundy, when domestic pets
were introduced in the 19th century by fishers and lighthouse keepers. Inadvertent introduction of rats on the
islands of the Atlantic coast of Nova Scotia also decimated the populations of burrow nesting petrels.) The
primary defense against mammalian predators is the selection of isolated, inaccessible breeding sites such as
offshore islands and steep cliffs.
It has long been thought that one of the effects of colonies is synchrony in the breeding cycles; that is, the
breeding activity of other pairs—copulating or courtship—stimulates breeding in nearby pairs. The result is
that most chicks are born around the same time, a circumstance that may confer some survival advantages. The
chicks also fledge around the same time, further reducing the risk of being picked off by ravens or gulls. The
potential predator is said to be “swamped” by the number of potential prey. It is also worth noting that there is
a higher survival rate for chicks born near the center of a colony rather than on the periphery, where they are
more vulnerable to predation. Being part of a colony may also help individuals find food, which for seabirds is
often far from the nesting site. Birds deprived of food may follow others that they observe successfully bring-
ing food back to the nest or that simply look more fit. Seabirds often have to travel farther for food than other
birds—up to 450 kilometers (280 miles) in the case of gannets. Birds seem to follow the successful fishers, just
as human fishers use seabirds as signs of where to set their nets or lines. The distances between the nest site
and the foraging area explain, in part, why breeding usually requires both the male and the female to be in-
volved in chick rearing—reaching the feeding grounds and collecting food takes more time than it does for
land birds. In addition, the need to defend a nest far from the foraging areas probably also led to monogamy,
which is predominant among seabirds. For the good of the chicks, someone has to stay home.
Colonial breeding has its drawbacks. It increases competition for food, especially among species such as
terns and gulls that forage close to the colony. The crowding also encourages the proliferation of microbes and
parasites and the transmission of disease. And crowding can lead to aggression or cuckoldry.
Seabirds are fundamentally different from other marine creatures, such as whales and fish, in that they breed
on land. Often their choice of a breeding site is a compromise between an area where oceanographic condi-
tions produce sufficient food and one that provides adequate protection from predators. To improve the effi-
ciency of feeding, they use oceanographic phenomena, such as fronts and upwellings, that concentrate prey.
Even so, birds often need to forage within a 35 to 50-kilometer (20 to 30-mile) radius of their colonies. Find-
ing a site well protected from predators, however, is not as easy as might be expected. The southern and east-
ern coasts of Newfoundland provide some suitable cliff sites, most notably at Cape St. Mary's, as does the
rugged coast of Labrador. The largest colonies, however, are restricted to islands off the coast.
