If you yourself can't breed, you can at least help your relatives with their offspring. Such altruistic behaviour occurs in long-tailed tits (Aegithalos caudatus, pictured), which Stuart Sharp and colleagues have studied to find out what cues enable a ‘helper’ to recognize kin. Their report appears elsewhere in this issue ( Nature 434, 1127–1130; 2005 ).
Adult long-tailed tits pair off and attempt to breed each year, but many don't succeed because of high rates of predation on the eggs or nestlings. The childless parents may then turn to assisting kin in feeding their brood — which makes sense in evolutionary terms but requires some form of recognition system. Long-tailed tits are not the greatest of vocalists. They sing infrequently but do have an individually characteristic contact call, the ‘churr’, which develops even before fledging and is retained in the adult bird.
The first part of Sharp and colleagues' research involved the playback to individuals of churr calls belonging to a close relative and a non-relative, and a further two trials in which the frequency of these calls had been tweaked. The responses of birds to the untweaked calls of relatives differed significantly from their responses to the other three calls. From this, the authors conclude that the churr call provides cues involved in kin recognition.
The most innovative part of the study, however, was an investigation into how much churr acquisition owes to nurture (learning) and nature (genetics). This took the form of swapping young birds between nests, so that adult birds were raising foster nestlings along with their true offspring. The churr calls of the fostered birds were the same as those of their nestmates, and unlike those of their biological siblings raised elsewhere — so it seems that the churr is in large part learned.
The pattern of helping observed in long-tailed tits is consistent with the use of this learned cue in the great majority of cases. But this kin-recognition system evidently isn't faultless: in 6% of cases, an adult helped unrelated nestlings. As the authors point out, recognition systems are rarely perfect.