Chiffchaff
Phylloscopus collybita
Grey-olive plumage, with a faint yellow streak over the eye.
Habitat: found in woodlands and hedgerows as well as parks and gardens.
Distribution: throughout British Isles, except far north and west Scotland. A summer visitor, although increasingly a resident particularly in southern England.
Audio
The chiffchaff has a short two tone song:
Not to be confused with
Willow warbler
It’s hard to tell apart from willow warbler with sight alone but listen to the song. Willow warbler has a longer more melodious song. (northeastwildlife.co.uk)