Parent-Chick Recognition in Colonial Nesters
photograph of Bank Swallows by Tom Grey	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.