Willow warbler – Phylloscopus trochilus
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.

The song
Recording by Eetu Paljakka from Xeno-Canto