In the preceding texts I have attempted to present a balanced and reasonably objective account of the often-divisive topic of ‘Siberian Chiffchaff’. Such complex issues cannot be dealt with adequately in a few words. Below, however, is a summary of some key issues - but this should be regarded as no more than a 'résumé'. Reference to the main text is essential if misunderstandings are to be avoided (or at least minimised).
As well as a lack of olive and yellow in appropriate tracts (see Svensson 1992), classic ‘Siberian Chiffchaffs‘ are characterised by distinctive hues of ‘tan-brown’ in the upperparts and a ‘rusty-buff‘ suffusion to supercilium, ear-coverts, sides of the breast and flanks.
The standard call of Siberian Chiffchaff differs from the familiar rising 'huit' of collybita / abietinus. It is a slightly mournful, nearly monotone 'eeep', predominantly even in pitch but descending and fading slightly at the end.
The standard song of Siberian Chiffchaff is much more varied and 'tuneful' than that of Common Chiffchaff. It rises and falls in pitch, with ascending modulations which are absent from abietinus / collybita song.
Among Siberian Chiffchaffs in the West Siberian Plain, individuals are found which closely resemble 'classic' tristis but have traces of 'misplaced' olive and yellow (see Svensson 1992). Such individuals are referred to as showing 'fulvescens' traits.
While the presence of limited ‘fulvescens’ traits may well arise from fractional introgression, the West Siberian Plain lies beyond the normal breeding range of abietinus i.e. here tristis and abietinus are fundamentally allopatric.
In a region around the Urals, the ranges of tristis and abietinus overlap i.e. they are sympatric and here widespread hybridization has been confirmed by 'whole genome' sequence data (Shipilina et al. 2017).
Shipilina et al. concluded from their studies in the sympatric and allopatric regions that natural plumage variation among tristis was limited, that genetic admixture in the sympatric zone is the driving force underlying trait variation (including song anomalies) and inferred that a 'fraction' of introgressed abietinus genes was the source of traces of misplaced yellow on some individual in the allopatric West Siberian Plain.
They noted that: ‘analysis of genomic composition of birds within the sympatric zone showed that several individuals express diagnostic plumage characters and perform diagnostic song despite harbouring the foreign mtDNA type and/or a considerable proportion of foreign nuclear alleles’.
Thus, individuals with a given morphotype and matching mtDNA might yet harbour 'mixed' alleles (e.g. a tristis morphotype with tristis mtDNA may still harbour a proportion of abietinus alleles).
Full trait data were available for only a limited sample but indicated mismatches between appearance and mtDNA were more likely in abietinus morphotypes than in tristis morphotypes.
This has led to suggestions that a Chiffchaff which 'looks like tristis is likely to actually be tristis' but this ignores the finding by Shipiiina et al. that such matching appearance and mtDNA may still mask mixed alleles in the nuclear genetic make-up.
As well as exhibiting intermediate appearance, many hybrids also employ conflicting song-type or 'mixed' song.
Marginal levels of 'misplaced' yellow and tristis-dominated 'mixed' song can both be difficult or impossible to discern in the field. In practice, distinguishing between 'classic' tristis and individuals with slightly intermediate traits may be challenging..
Van den Berg et al. (2009) reported that tristis-style Chiffchaffs encountered on migration in Kazakhstan exhibited 'wide plumage variation' yet called and sang 'exclusively' like tristis (though this was not confirmed by comprehensive sound-recording sonogram analysis). This was interpreted by van den Berg et al. (2009) as indicating extensive variation among thoroughbred tristis but the findings of Shipilina et al. (2017) lead to a different interpretation : that 'wide plumage variation' results from genetic admixture and that neither tristis calls nor song exclude hybrids.
In a study in the Netherlands (de Knijff et al. 2012) a number of Chiffchaffs trapped and diagnosed in the field as abietinus on the basis of appearance proved to harbour tristis mtDNA. This also led to suggestions that thoroughbred tristis were more variable morphologically than traditionally believed. Some Chiffchaffs diagnosed by their ringers as abietinus were in fact reasonable candidates for tristis (based upon published photos) but others were indeed 'intermediate', with misplaced olive and/or yellow. Marova et al. (2017) noted that subsequent genetic studies in the sympatric zone suggested such anomalous Chiffchaffs were more likely to be hybrids.
Thus, 'classic' morphotype and 'classic' vocalisations remain the foundations for identification of all candidate 'Siberian Chiffchaffs' encountered extralimitally. No character is diagnostic in isolation.
It is important to recognise that absolute adherence to 'classic' morphology can only be guaranteed by examination in the hand and the findings of Shipilina et al. indicate it is possible that such individuals may still harbour 'foreign' alleles.
However, to accept and document only that small percentage of candidates which is trapped, or even demand 'whole genome' genetic data, would seriously misrepresent the status of Chiffchaffs which reach western Europe from Siberia.
From a population perspective, the levels of genetic mixing in the sympatric and allopatric regions are on altogether different scales. It is preferable to recognise this by maintaining a distinction via the colloquial terms 'riphaeus' (for first generation hybrids and early backcrosses from the sympatric zone) and 'fulvescens' (restricted to individuals with fractional introgression found in the allopatric West Siberian Plain).
These are colloquial terms and have no taxonomic implications - they should not be italicised and should always be placed in inverted commas.
Clearly, first generation hybrids and early backcrosses from the Urals region ('riphaeus' types with well-mixed genes) cannot be regarded as Siberian Chiffchaffs.
Conversely, trace levels of introgression (and slight 'fulvescens' traits sensu stricto), among individuals across the allopatric West Siberian Plain, need not disbar diagnosis as tristis. (See, for example, general discussions of the 'genic view' of species and 'speciation genes' in Wu (2001) and 'introgression' and 'semipermeable species boundary' in Hybridization, Introgression, and the Nature of Species Boundaries (Harrison and Larson 2014).
Despite these significant differences in the population levels of genetic mixing, there will be individual variation in appearance (e.g. extent of yellow) among 'riphaeus' types from the sympatric zone and 'fulvescens' sensu stricto types from the West Siberian Plain. Shipilina et al. reported that, in the sympatric zone, even Chiffchaffs with the appearance of 'pure tristis' and with tristis mtDNA can have abietinus alleles within their nuclear genetic make-up. No non-invasive criteria for tristis will be infallible. Inevitably, there will remain a 'margin of error'. Where preferred, the phrase 'showing the characters of tristis' might be appended.
Key assumptions are that: (a) the proportion of tristis candidates reaching western Europe from the sympatric zone is relatively small (this remains to be demonstrated and, conceivably, may be false); (b) Chiffchaffs with slight 'fulvescens' traits found across the West Siberian Plain (a very likely source for birds reaching the west) have only 'fractional' introgression and are reasonably diagnosed as Siberian Chiffchaffs.
Pragmatic criteria for identification in the field will be based on these assumptions and founded on the traditional 'Svensson criteria' while recognising that, in the field, slight 'fulvescens' traits (e.g. traces of misplaced yellow) may go undetected.
When evaluating the status of tristis in Britain, using 2008 as a sample year, the BBRC 'tristis panel' adopted criteria based upon such conservative yet pragmatic criteria. Song and call were also given due consideration but not deemed to be unequivocally diagnostic (Stoddart 2008, Dean et al. 2010). In order to exclude likely hybrids and variant abietinus, more-evidently intermediate-looking individuals and enigmatic 'grey and white Bonelli's-like' individuals (see main text) were excluded, both differing too significantly from the 'classic' criteria. There was an implicit assumption that individuals from the sympatric zone with ‘pure’ tristis morphology would be invariably thoroughbreds. This is now known to be false but, pending the outcome of nuclear DNA studies of tristis candidates reaching western Europe, the retention of 'pragmatic' criteria seems justified.
Based upon the criteria adopted, the panel concluded that 'Siberian Chiffchaff ' was a scarce rather than rare visitor (49 of 57 submitted reports were accredited while a 'scaled' estimate from newslines' reports suggested around 120 during 2008). 'Scarce migrant reports', published annually in British Birds, indicate comparable or somewhat enhanced numbers in subsequent years.
Field identification of 'Siberian Chiffchaff' remains challenging. It requires very careful evaluation of vocalisations and morphology and an acceptance that 100% failsafe diagnosis is unachievable.