Wilson's Storm Petrel

Wilson's Storm Petrel (Oceanites oceanicus) is a small dark storm petrel with a white rump and long legs whose yellow webbing can be conspicuous at close range. Body shape, bill structure, plumage pattern and voice provide the strongest field characters. Age, sex and season may alter its appearance, so identification should combine several features and careful direct field comparisons with similar species in the same habitat (Wikipedia, general species account).

It ranges widely across the world's oceans, pattering at the surface for planktonic prey, and breeds on Antarctic and subantarctic islands. Its movements and social behaviour vary through the year, with breeding birds generally more dispersed and non-breeding birds gathering where food is concentrated. Japanese records place it within the country's documented avifauna as a resident, migrant or visitor (Wikipedia, general species account; Wildlife of Japan occurrence records).