Arctic Warbler

Arctic Warbler (Phylloscopus borealis) is a greenish leaf warbler with pale underparts, a prominent eyebrow and one or two narrow wing bars. 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 breeds in birch and mixed forest near water across the northern Palearctic and Alaska, gleans insects from leaves and undertakes a very long migration to Southeast Asia for winter. 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).