Use of cloacal sexing techniques in mark-recapture estimates of breeding population size in Wilson's stormpetrel Oceanites oceanicus at South Georgia

Summary

Many stormpetrel species breed in habitats where their populations cannot be estimated by direct counts of burrows or birds; mark-recapture experiments have been confounded by the presence of many wandering non-breeders. With a population of Wilson's Stormpetrel Oceanites oceanicus at Bird Island, South Georgia, we tried to estimate the proportion of breeding females in samples obtained during a mark-recapture experiment. These were identified by measurements of the cloaca, which greatly enlarges at egg-laying. A concurrent experiment with individually marked birds determined that breeding females could be discriminated from males and non-breeders for c 30 days after laying. The technique is probably applicable to other petrels, though it will work best with those that lay most synchronously. The overall population estimate was 4841–5515 birds (SE 856–1417); estimates of breeding females gave a population of 2300 paris early in the incubation period and 1400 pairs near hatching.