© Sakkarin Sansuk
© Zhen niu
© Akekachoke Buranaanun
© Wich’yanan Limparungpatthanakij

Alström's Warbler Phylloscopus soror

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Sprightly, small warbler that poses a bevy of identification issues. Dull olive above and yellow below, with a bright eyering and a gray crown bound in by black stripes. Extremely similar to closely-related Martens’s, Gray-crowned, White-spectacled, and Bianchi’s Warblers; often impossible to identify if seen poorly in where range overlap is possible. Weak or absent wingbar distinguishes this species from Whistler’s, Bianchi’s, and White-spectacled warblers. Slightly larger-billed and shorter-tailed and with more restricted white in outer tail feathers than Martens’s and Gray-crowned warblers. Breeding altitudinal range (600–1500 m) can be helpful: does not overlap with higher-breeding Bianchi’s Warbler (1760–3100 m in China) and minutely with Martens’s Warbler (1200–2300 m). Overlap more extensive with Gray-crowned Warbler (120–1830 m in China) and White-spectacled Warbler (1000–1750 m in China). Simple, short song can be an identification clincher: a brief “che-witchy-chew-witchy-chew”, lacking the trilled notes of Martens’s, Gray-crowned, and White-spectacled warbler, and higher in pitch than Bianchi’s Warbler. Forages low, often in dense growth, of temperate evergreen forests, descending during the non-breeding season into forested lowlands, where overlap with non-singing similar species can make identification nearly impossible.