Shipilina et al. 2017 have made progress with the tricky problem of the relationship between Chiffchaff and Siberian Chiffchaff (better treated as subspecies rather than distinct species), and in particular with the status of fulvescens (an intergrade rather than a valid subspecies). Some of their findings are:
Hence, many birds that appear as obvious, distinct abietinus or tristis can harbor a genetic set-up that is a mix between species or even almost completely fixed for foreign alleles. We conclude from the combined analyses of phenotypes, mtDNA, and genome-wide SNPs in both allopatry and sympatry that extensive historical and ongoing gene flow between the subspecies have resulted in a transition zone of morphotypes and song types.
In this study, we combine genetic data with morphology and vocalizations across a hybrid zone between European (abietinus) and Siberian (tristis) chiffchaff. Our results indicate that the overall genetic differentiation between subspecies is low despite considerable phenotypic divergence. A large proportion of birds in the hybrid zone exhibit intermediate phenotypic characters and a mix of genetic ancestry, indicating extensive ongoing and past gene flow. Patterns of phenotypic and genetic variation vary between the northern and the southern part of the hybrid zone, probably reflecting differences in population densities and/or relative age of secondary contact. The data indicate that birds showing intermediate characters very likely represent individuals resulting from recurrent backcrossings and introgression of (predominantly) abietinus alleles into a tristis genomic background, and the previously described subspecies “fulvescens” is therefore not to be considered a distinct taxon. The data also point to the difficulties in identification of specific individuals based on phenotypic characters alone.
I have previously discussed this topic (in a different context) here: https://rutlandbutterflies.wordpress.com/tag/siberian-chiffchaff/ How much difference do the recent findings make for field observers? My interpretation is that not much has changed: if birds look and/or sound like tristis, they probably are tristis; if they show any anomalous characters, they are likely to be intergrades. Although Shipilina et al. concentrated on intergrades, it is worth remembering that intergrades are mainly found in a relatively narrow zone and are much less numerous than the parent taxa, which accords with my experience of migrant and wintering birds in Britain. Also, in my experience most migrant and especially wintering Chiffchaffs tend to be silent, so call-based identifications will leave (too) many unidentified. What remains unresolved is how we can identify abietinus in the field.
The bird in the photograph (which I did not see in the field) shows several characters of tristis, and had tristis DNA (per FIBO and Martin Collinson). However, note the yellowish tones in the fore-supercilium and along the flanks which are more striking than we associate with ‘classical’ tristis; do they betray introgression, or are these within the range of variation of tristis? We already know that tristis is a scarce migrant rather than a vagrant, so a ‘liberal’ position is likely to deliver a more complete picture of status. It is certainly preferable to use the label ‘tristis or intergrade’ rather than ‘not proven tristis’. It was photographed on Fair Isle on 4 October 2016.