The willow warbler (Phylloscopus trochilus) is a very common and widespread leaf warbler which breeds throughout northern and temperate Europe and Asia, from Ireland east to the Anadyr River basin in eastern Siberia. It is strongly migratory, with almost all of the population wintering in sub-Saharan Africa.
It is a typical leaf warbler in appearance, 11–12.5 cm long and 7–15 g weight. It is greenish brown above and off-white to yellowish below; the wings are plain greenish-brown with no wingbars. Juveniles are yellower below than adults. It is very similar to the chiffchaff, but non-singing birds can be distinguished from that species by their paler pinkish-yellow legs (dark brown to blackish in chiffchaff), longer paler bill, more elegant shape and longer primary projection (wingtip). Its song is a simple repetitive descending whistle, while the contact call is a disyllabic ‘hoo-eet’, distinct from the more monosyllabic ‘hweet’ of chiffchaffs.
Three subspecies are accepted, with a partly clinal reduction in green and yellow plumage tones from west to east, with central birds browner and easternmost birds predominantly greyish:
Phylloscopus trochilus trochilus (Linnaeus, 1758). Breeds Europe (from the Pyrenees and Alps northward) except northern Scandinavia, winters west Africa.
Phylloscopus trochilus acredula (Linnaeus, 1758). Breeds northern Scandinavia east to western Siberia, winters central Africa.