Siberian Chiffchaff

The Siberian chiffchaff is a dull bird, grey or brownish above and whitish below, with little yellow in the plumage, and its buff-white supercilium is often longer than in the western subspecies. It has a higher-pitched suitsistsuisit song and a short, high-pitched cheet call. It is usually considered a subspecies of the common chiffchaff but is sometimes treated as a full species because of its distinctive plumage and vocalisations, which resemble those of the form sindianus. (Wikipedia, general species account)

It breeds in Siberia east of the Pechora River and winters in the lower Himalayas. It is also regularly recorded in western Europe in winter, and the numbers involved are probably underestimated because of uncertainty over identification criteria and recording policies. British records in the 1950s and 1960s were originally taken as greenish warblers until they were reviewed in the 1980s. (Wikipedia, general species account)