Booted Warbler

The booted warbler is a small warbler, similar in size to a common chiffchaff. Its plumage is undistinguished, plain greyish-brown on the upperparts and paler on the underparts, with a clear pale supercilium and a short bill with a dark tip. It gives the impression of a short-winged, rather pot-bellied warbler with a square-tipped tail. This species and Sykes's warbler are challenging to identify. (Wikipedia, general species account)

It breeds from central Russia to western China and migrates to winter in the Indian subcontinent as far south as Sri Lanka. Over recent decades it has expanded its breeding range westward, and it now nests as far west and north as Finland. It is a regular vagrant to western Europe, with over 180 records in Great Britain up to 2023. (Wikipedia, general species account)