Kin recognition: function and mechanism in avian societies.

Cooperative behaviour resulting from kin selection is widespread among animals and the ability to recognize and discriminate between kin and non-kin is a critical element in kin selection theory. Current evidence suggests that associative learning is the most likely mechanism of kin discrimination. However, surprisingly, there have been no experimental studies of the putative 'associative-learning period', the likely recognition mechanisms enabling fine discrimination between close and distant kin of similar familiarity, whether generic or individual cues are employed in kin recognition, and how recognition ability varies at different stages of a species' life history. Comparative studies of kin recognition and discrimination in cooperative and noncooperative species are also needed to shed light on the adaptive value of helping behaviour and to identify key factors in the evolution of cooperation.