Breeding Status and Sex Ratio of Ancient Murrelets Captured in the Nocturnal At-Sea Congregation Off Teuri Island, Japan

We examined brood patch development, sex ratios, and morphological traits of 30 Ancient Murrelets Synthliboramphus antiquus captured from the nocturnal at-sea congregation off Teuri Island, Hokkaido, Japan during late May to early June 2016. Molecular sexing determined that 12 of the murrelets were males and 18 were females. The proportions of murrelets with brood patches (an indicator of breeding status) were similar for males (58%) and females (56%). Breeding murrelets were heavier than non-breeding murrelets. Males had deeper bills and longer heads than females, but their weights, wings, culmens, tarsi, bill widths, and skull widths were similar.