About Phylloscopus proregulus (Pallas, 1811)
Pallas's leaf warbler (Phylloscopus proregulus) is one of the smallest warblers, with a large head and short tail. It measures 9–10 centimetres (3.5–3.9 in) in length and weighs 4–7 grams (0.14–0.25 oz), making it slightly smaller than a yellow-browed warbler and barely larger than a goldcrest. It has greenish upperparts and white underparts, with striking prominent markings: pale yellow double wingbars on the wing covert feathers, bold yellow supercilia, a central yellow crown stripe, and a lemon-yellow rump. Its bill is blackish-brown with a yellowish tinge on the cutting edges and the base of the lower mandible, its iris is brown, and its legs are brown with a green or greyish tinge. While the yellow rump is obvious when the bird is low in vegetation or hovering, it can be hard to see in other positions. In Asia, this species can be distinguished from its former subspecies by yellower head stripes, wingbars, and throat, as well as different vocalisations. Other yellow-rumped Asiatic warblers resemble Pallas's leaf warbler: buff-barred and Brooks's leaf warblers are larger, duller green on the upperparts, less strongly marked, and have buff and white wing bars respectively, rather than yellow. Ashy-throated warblers have grey head markings, face, and throat, and pale yellow underparts. The sexes of Pallas's leaf warbler have similar plumage. Non-breeding birds have somewhat brighter green upperparts and broad, bright fringes on their flight feathers. Juveniles resemble adults, but have a brown tinge on the upperparts, greyish-white underparts, and a duller supercilium. Adults undergo a complete post-breeding moult in August or September before migrating south. Juveniles and pre-breeding adults have a partial moult in March or April, which replaces all body plumage and some tail feathers. The song of Pallas's leaf warbler is sung from a concealed perch near the top of a tall tree. It is strong and prolonged, made up of a medley of whistles, transcribed as tirrit-tirrt-tirrit-terchee-choo-choo-chee-chee-chee or similar. Some phrases are reminiscent of a canary, and trills are interspersed throughout. The song lasts 2–4 seconds, and can be heard both from breeding birds and on the winter quarters. The call is a short, soft dju-ee. In contrast, the former subspecies have quite different songs that include sustained rattles lasting for several seconds, or sometimes minutes. Their calls are typically sharp and monosyllabic. Pallas's leaf warbler breeds in Siberia from the Altai Mountains east to the Sea of Okhotsk, northern Mongolia, northeast China, and possibly North Korea. It is strongly migratory, and winters mainly in subtropical south China, northern Thailand, and other parts of northeastern Indochina. It is rare but occurs annually in Japan. This species breeds in coniferous taiga forests including fir, spruce, pine, and larch, or in mixed forest that includes willow, alder, and a high percentage of conifers. In southern and far-eastern Russia, its breeding range extends from sea level up to 1,500–1,700 metres (4,900–5,600 ft). In winter, it uses a wider range of habitats, including broadleaf forest, scrub, and conifers, and can be found in river valleys down to sea level.
