The Pallas's Warbler or Pallas's Leaf Warbler (Phylloscopus proregulus) is a leaf warbler which breeds in southern Siberia (from Novosibirsk Oblast east to Magadan Oblast), northern Mongolia, and northeastern China. It is strongly migratory and winters mainly in subtropical southern China and northeastern Indochina, but also in small numbers in western Europe.
It is one of the smallest warblers; at 9–10 cm long and 4–7 g weight it is slightly smaller than a Yellow-browed Warbler and barely any larger than a Goldcrest. Overall, it has greenish upperparts and white underparts, but is very striking, with prominent pale yellow wingbars on the tips of the greater and median coverts, bold yellow supercilia and central crown stripe, and a lemon-yellow rump. It is not shy, but its unobtrusive arboreal lifestyle makes it difficult to observe. It is constantly in motion, and often hovers briefly like a Goldcrest. Its song is a medley of whistles, with some phrases reminiscent of a Canary ; the call is a short 'chweey'.
It breeds in coniferous taiga and mountain forests, and like most warblers, is insectivorous. The nest is built in a tree, usually next to the trunk at 0.5–10 m above ground; four to six eggs are laid, hatching after 12–13 days, with the chicks fledging when 12–14 days old.
In winter, it uses a slightly broader range of habitats, including broadleaf forest and scrub as well as conifers. Although the vast majority winter in southern China south of the Yangtze River (where they are present from October to April), small numbers also regularly winter in western Europe.
These arrive in Great Britain in October to November after a 5,000 km migration from the western end of the breeding range, about the same distance that they would need to fly to reach the normal wintering areas in southern China. Exact numbers in this population are unknown, but up to 300 per year have been found arriving in Great Britain in autumn; given their unobtrusive behaviour, this is probably only a fraction of the total. In the past widely considered to be vagrants, these birds are now thought to be undertaking a normal regular migration, taking advantage of the mild oceanic climate winters on the western fringes of Europe for wintering.
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