Just when you think they can be identified.....
Above; tristis 'Siberian' Chiffchaff a calling individual with typical sad, flat 'piii' call. I watched as it fed confirming pretty much all of the pro tristis features, brownish grey upperparts with no olive except slightly on edges of secondaries, primaries and tail, slightly rusty cheeks black legs only slight pale base to bill, brownish grey shoulders and flanks and buffy tone to big supercilium and throat. It responding brilliantly to tristis song playback with wing quivering and fluffed cloaca feathers and calling (as one did last year with a chit- chit call) so presumably a female.
Then... A second bird appeared high in the same willow also wing quivering but his time to the other bird rather than the song, it attempted to mount the tristis, so I assumed another tristis but no... It moved down lower to show a dirty brownish olive upperparts and the same colour but almost streaky sides of chest contrasting with an almost grey/white belly and pale yellow undertail coverts. The legs were browner and more pale at the base of the bill. Supercilium not nearly so strong and showing a good eye ring. 10 minutes later when the trisits had moved on it began to sing, to start with just like a normal chiffchaff then it broke into tristis song to finish off - it did this three times! I played the tristis song back to it and it came over to investigate but not wing quivering, it called regularly - an abietinus type call short sharp almost monosyllabic 'seu'. If I had to put a bet on it it would be for an abietinus 'Scandinavian' Chiffchaff. But a mixed singer complicates things!?
(there were at least 5 collybita 'normal' Chiffchaffs also in same area none of these were at all interested in the tristis song playback or the wing quivering birds. They did respond and wing quiver when I played collybita song playback. The tristis or other bird ignored the collybita song playback completely)