The Willow Warbler (Phylloscopus trochilus) is a very common and widespread 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 bird of open woodlands with trees and ground cover for nesting, including most importantly birch, alder, and willow habitats. The nest is usually built in close contact with the ground, often in low vegetation. It's insectivorous. In northern Europe, it is one of the first warblers to return in the spring though is later than the closely related Chiffchaff (Phylloscopus collybita).
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. 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 legs, longer paler bill, more elegant shape and longer primary projection. 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.