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Photo: Mike Pennington · CC BY-SA 2.0 (May be cropped or resized)
Phylloscopus borealis
Diet
Invertebrates
Habitat
Birch or mixed birch forest near water; nest on the ground in a low shrub
Range
Breeds in Fennoscandia, the northern Palearctic and Alaska; the entire population winters in Southeast Asia
The Arctic warbler is a widespread leaf warbler of birch or mixed birch forest near water throughout its breeding range in Fennoscandia and the northern Palearctic, with a foothold in Alaska. It is greyish-green above and off-white below, and its single wing bar distinguishes it from most similar species except the greenish warbler, though it is larger than that species with a heavier, dagger-like bill with a dark tip to the lower mandible (Wikipedia, general species account).
It is strongly migratory, the entire population wintering in Southeast Asia, one of the longest migrations of any Old World insectivorous bird. The nest is on the ground in a low shrub and the song is a fast trill (Wikipedia, general species account).