The garden warbler – Sylvia borin
is a common and widespread small bird that breeds in most of Europe and in the Palearctic to western Siberia. It is a plain, long-winged and long-tailed typical warbler with brown upperparts and dull white underparts; the sexes are similar and juveniles resemble the adults. Its two subspecies differ only slightly and interbreed where their ranges overlap. Due to its lack of distinguishing features, this species can be confused with a number of other unstreaked warblers. The garden warbler’s rich melodic song is similar to that of the blackcap, its closest relative, which competes with it for territory when nesting in the same woodland.

The garden warbler is 14 cm (5.5 in) long with a 7.6–8.4 cm (3.0–3.3 in) wing length.[10] The weight is typically 16–22 g (0.56–0.78 oz), but can be up to 35.5 g (1.25 oz) for birds preparing to migrate.[12] It is a plain, long-winged and long-tailed bird with unstreaked olive-brown upperparts and dull white underparts. It has a whitish eyering and a faint pale supercilium, and there is a buff wash to the throat and flanks. The eye is black, the legs are bluish-grey and the strong bill has a grey upper and paler grey lower mandible. The male and female are indistinguishable by external appearance including size. Juveniles have a looser plumage than an adult, with paler and greyer upperparts and a buff tone to the underparts. The eastern subspecies S. b. woodwardi is slightly larger and paler than the nominate form with a greyer tone to the upperparts and whiter underparts. The subspecies are hard to distinguish visually where they occur together in Africa, but a wing length greater than 80 mm (3.1 in) confirms S. b. woodwardi when birds are trapped.

Source:Wikipedia

It sounds like this
Recording from Xeno canto by Jorge