Immigration prevents inbreeding in a growing colony of a long-lived and philopatric seabird

Mating between close relatives can have deleterious effects on reproductive success or offspring fitness, which should favour the evolution of active or passive inbreeding avoidance mechanisms. In birds, evidence for active inbreeding avoidance by kin-discriminative mate choice is scarce; many studies describe random mating in relation to kinship and thus support passive inbreeding avoidance by natal dispersal. However, most studies were conducted in island populations of short-lived passerines with fast alternation of generations. In this study, we present inbreeding estimates based on pedigree data from a 16-year study in a coastal colony of Common Terns Sterna hirundo, a long-lived seabird with delayed sexual maturation and low rates of extra-pair paternity. Incestuous mating was rare (four of 2387 pairs), even if partially accounting for incomplete pedigrees. Although the average relatedness of observed pairs was lower than would be expected from random pairing, the inbreeding coefficient did not differ from random mating. Hence, we found no clear evidence for active inbreeding avoidance by kin-discriminative mate choice, and the low level of inbreeding seems to be related to the high immigration rate in the colony and thus to be maintained passively by dispersal.