Willow Warbler
Phylloscopus trochilus
The willow warbler is a typical leaf warbler in appearance, 11–12.5 cm (4.3–4.9 in) long and 7–15 g (0.25–0.53 oz) 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).
ID text from Wikipedia, CC BY-SA 4.0

Wood Warbler
Phylloscopus sibilatrix
The wood warbler is 11–12.5 cm long, and a typical leaf warbler in appearance, green above and white below with a lemon-yellow breast. It can be distinguished from similar species, like the chiffchaff P. collybita and the willow warbler, P. trochilus by its yellow supercilium, throat and upper breast, pale tertial edges, longer primary projection, and by its shorter but broader tail. It is a summer visitor to the United Kingdom, seen from April until August. It has declined there in recent years. Voice: It has two song types, often (but not always) given alternately; a high-pitched fluid metallic trill of increasing tempo lasting 2–3 seconds, and a series of 3 to 5 descending piping notes of lower pitch piüü-piüü-piüü. The contact call is a soft piping note, similar to the second song type, but shorter and given singly, piü.
ID text from Wikipedia, CC BY-SA 4.0

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