No evidence for inbreeding avoidance through active mate choice in red-billed gulls

Except for cooperative breeders, most studies on wild birds have failed to find evidence for inbreeding avoidance via kin discriminative mate choice. This, together with evidence for kin avoidance through dispersal, has led to the general view that dispersal is often a sufficient inbreeding avoidance mechanism and active discrimination through mate choice is unnecessary. Yet, the study of inbreeding avoidance in the wild is difficult and long-term studies of pedigreed wild populations can provide important insights. We studied the occurrence of inbreeding avoidance in a highly philopatric red-billed gull (Larus novaehollandiae scopulinus) population subject to an individual-based field study since 1958 in Kaikoura, New Zealand. Despite a wealth of breeding and pedigree data, we did not observe a single inbred pair. This observation was a small but significant deviation from the expectation under the null hypothesis of random mating when we looked at annual breeding attempts, suggesting inbreeding avoidance. However, the difference disappeared when we examined pair bonds rather than annual breeding attempts. Our results are consistent with the expectation that close inbreeding occurs rarely in large random-mating populations. They also demonstrate how mating systems, in this case long-term monogamous pair bonds with sex differences both in the age of first breeding and breeding dispersal at natal subcolonies, can reduce the likelihood of inbreeding.