The willow warbler (Phylloscopus trochilus) is a very common and widespread leaf warbler which breeds throughout northern and temperate Europe and the Palearctic. It is strongly migratory, with almost all of the population wintering in Sub-Saharan Africa.
]Willow Warblers are summer visitors, appearing in the UK from about late March or early April after having spent the winter in Africa, south of the Sahara. Their migration is further than that of the Chiffchaff and this is why they have slightly longer wings.
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. Like most Old World warblers (Sylviidae), this small passerine is insectivorous. In northern Europe, it is one of the first warblers to return in the spring though is later than the closely related chiffchaff.
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.