This winter at least four Chiffchaffs are wintering at Geeston STW, where a Siberian Chiffchaff (tristis) was present last winter (see https://rutlandbutterflies.wordpress.com/tag/siberian-chiffchaff/ ).
Three of them look like typical collybita (below), but one is different and either confusing or interesting depending on your point of view (left). Its upperparts are noticeably more grey/brown, the yellowish tones in the supercilium are much more subtle, and the underparts are whiter with pale yellow undertail-coverts.
Its appearance does not correspond with our understanding of tristis, yet it is sufficiently different from typical collybita to command attention. I have seen a few birds like this previously, but remain unsure what they are. They might be intergrades, or they might be Scandinavian Chiffchaffs (abietinus), the status of which needs re-evaluation.
Any informed comments will be welcome.