For reasons unknown, no other authorities to date have treated tristis as a separate species even though the differential of mitochondrial DNA of 1.7-2.0% is much higher than the 0.4% divergence of Pine Bunting vs Yellowhammer. Siberian Chiffchaff is highly distinct from the other chiffchaffs in many respects including plumage and most importantly songs and call. I am pleased to report however a new paper by Arnoud B van den Berg & The Sound Approach in Dutch Birding 31: 79-85, documenting further results of research in this species.
They have concluded that Siberian Chiffchaff shows a slight morphological variation, becoming browner and less olive from west to east, and that some western individuals can approach some abietinus in appearance. Arend Wassink has spent a lot of time studying tristis in southern Kazakhstan and has concluded that any chiffchaff producing a typical tristis call SHOULD be a tristis, providing that plumage and bare-part coloration is within the typical variation of that species.
There is no genetic proof that western tristis are hybrids/intergrades and therefore more closely related to abietinus than to eastern tristis. Although just a few birds were sampled, Helbig and others in 1996 only found evidence of gene flow between nominate collybita and abietinus, and NOT between tristis and abietinus. Therefore, the idea that there is a wide zone of hybridization between tristis and abietinus (a theory put forward by several interested commentators on the complex) must be regarded as hypothetical. It is presumably based upon the cline in colour within the range of tristis, or perhaps upon the incidence of mixed-song which, by definition, is more likely to occur in western areas where abietinus may turn up.
Arend Wassink has put together an excellent selection of images taken in Kazakhstan showing the variations within tristis plumage (taken of birds in the hand). Classic individuals are those as plumaged in plate 71 (page 80) and to which the UK400 Club uses as a baseline for acceptance.