Tristram's Storm Petrel

Tristram's Storm Petrel (Hydrobates tristrami) is a relatively large, all-dark storm petrel with long wings, a forked tail and no white rump. 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 over the north-western Pacific, picking plankton, small fish and squid from the sea surface and nesting in burrows or rocky cavities on remote 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).