In
spite of the apparent chaos of seabird nesting colonies,
returning adults invariably manage to locate and feed only
their own offspring, indicating that some form of parental
recognition of chicks (or vice versa) must exist. The timing
and development of recognition, as well as whether it is
accomplished by parent or chick or both, have been explored
in species of shearwaters, penguins, gulls, terns, alcids,
and swallows. These studies have all shown that recognition
develops only in species in which circumstances, such as the
young wandering away from the nest or being gathered in
communal groups ("creches") for feeding, could lead to
confusion. The onset of recognition coincides with the time
when young of different broods begin to mingle. Where such
intermixing does not occur (such as in Manx Shearwaters that
nest individually in isolated burrows), researchers have
found no evidence of recognition between parents and their
chicks (the parents do, however, recognize their burrows or
nest sites). Many investigations have
employed the technique of exchanging broods at different
ages to determine whether recognition exists and how it
occurs. Among gulls and terns, the age at which recognition
develops is related to the timing of young leaving the
vicinity of the nest. For example, in the ground-nesting
Sooty Tern, adults reject strange chicks that are more than
4 days old. The tree-nesting Brown Noddy does not
discriminate between its own and strange chicks until about
14 days, which is the age when young leave the nest.
Black-legged Kittiwakes nest on cliff ledges, and young do
not mix until they fledge; adults do not discriminate
between their own and other chicks. In contrast,
ground-nesting Herring Gulls reject foreign chicks beginning
at about 5 days, but in cliff-nesting populations, where
young leave the nest later, adults will still accept
transfer chicks that are one to two weeks old. Franklin's
Gulls, with widely spaced floating nests, do not
discriminate among chicks less than one to two weeks
old. Development of recognition
has been most thoroughly studied in the Laughing Gull by
ethologist Colin Beer. Chicks remain close to the nest for 3
or 4 days, and although they recognize their parents' calls
beginning at I to 3 days, their discriminatory ability
becomes much sharper starting at 5 to 6 days. Learning
occurs in stages as the young gulls are exposed to different
adult calls. Adults identify their chicks by the response
that their calls elicit rather than by the calls of the
chicks. Similarly, Ring-billed Gulls recognize their chicks
by sight instead of by their vocalizations. In species with altricial
young, the timing of recognition is also correlated with the
potential for confusion with young from different broods.
Adult Common Murres nesting on crowded cliff ledges can
recognize their own chicks shortly after hatching; the
chicks learn their parents' call while still in the egg! For
most altricial species, the critical time requiring
discrimination comes at fledging. Young of the colonial Bank
Swallow fledge at 18-19 days. Until then, parents need
simply return to the correct burrow to ensure that they are
feeding their own young. Experiments confirm that exchanged
chicks are readily accepted until they reach 15 days of age,
when their begging calls are replaced by brood-specific
"signature" calls that the adults use to discriminate
between their own and other young. Why is the development of
parent-offspring recognition delayed until shortly before it
is required? The likely answer is that there is an
evolutionary cost involved in recognition prior to the onset
of chick mobility. Not only would earlier recognition be
superfluous for the purpose of rejecting alien chicks, it
could even lead to evicting one's own chick by
mistake! SEE: Vocal
Functions;
Vocal
Development;
Coloniality;
Creches. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.