The Effect of Siblings on Nest Site Homing by Common Tern Chicks: A Benefit of Kin Recognition

Abstract

-Studies of kin recognition in birds have rarely tested its adaptive value. We tested whether sibling recognition helps Common Tern (Sterna hirundo) chicks find their nests. The ability to discriminate the natal nest from neighboring nests is critical to survival in chicks of ground-nesting, colonial birds, such as the Common Tern. We temporarily moved 70 four-day old chicks 1 m from their nests and compared their ability to return home across the following treatments: siblings in natal nest, no chicks in nest, siblings in neighboring nest, and non-siblings in test chick's nest. With a sibling in the natal nest, test chicks returned within 20 min in 14 of 20 trials. In all other treatments, fewer than half of the chicks returned. When chicks did return, they typically did so within 5 min. Several additional factors may affect nest site homing, such as parental behavior, aggression by neighboring adults, and habitat type. To control for these factors, we restricted the dataset for analysis to 27 nests by excluding chicks led back to the nest by a parent, chicks attacked by neighboring adults, and nests on a rocky beach (most experiments were performed in grassy habitat). In the restricted dataset, seven of nine chicks with siblings in the nest returned. Two of five returned in the empty nest treatment, none of six with siblings at a neighboring nest returned, and one of seven with non-siblings in the nest returned. There was significant variation among treatments in the final distance of test chicks from their nests. Siblings appeared to affect nest site homing, even though parents usually returned quickly to the nest and were least likely to search for displaced chicks if siblings were home. We also tested the ability of singletons to return to the nest, using an identical protocol. Singletons returned to the nest in only two of 14 trials, in none of the six cases when the dataset was restricted. Received 15 September 2000, accepted 24January 2001.