The preceding texts attempt 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.
In a region around the Urals, the ranges of tristis and abietinus overlap i.e. they are sympatric.
In this region, individuals with intermediate morphology and 'mixed song' are encountered and widespread hybridization, with significant levels of genetic admixture, has been confirmed by 'whole genome' sequence data (Shipilina et al. 2017).
Across the West Siberian Plain, east of the region of sympatry and hybridization, Siberian Chiffchaffs are sometimes found which, while closely resembling 'classic' tristis, nevertheless have traces of 'misplaced' olive and yellow (see Svensson 1992).
The West Siberian Plain lies beyond the normal breeding range of abietinus i.e. here tristis and abietinus are fundamentally allopatric.
Shipilina et al. (2017) concluded from their studies in the sympatric and allopatric regions that natural plumage variation among thoroughbred tristis was limited, that genetic admixture in the sympatric zone is the driving force underlying trait variation (including song anomalies) and that a 'fraction' of introgressed abietinus genes was the source of traces of misplaced yellow on some individuals in the allopatric West Siberian Plain.
In view of these very different levels of genetic admixture, it is here advocated that hybrids of mixed phenotype arising in the direct region of hybridization around the Urals are best referred to colloquially as 'riphaeus' types.
Individuals in the basically allopatric West Siberian Plain yet exhibiting traces of 'misplaced' olive and yellow are best referred to colloquially as 'fulvescens'.
The term 'fulvescens' should be confined to such individuals and not applied to individuals with any level of genetic admixture.
Although the terms 'riphaeus' and 'fulvescens' are merely colloquial and have no taxonomic standing per se, they facilitate discussions of the appropriate taxonomic treatment of populations with differing levels of genetic admixture. Highly admixed genetically (F1 hybrids and early backcrosses), 'riphaeus' cannot be labelled as tristis. With only fractional admixture (low level introgression), a case can be made for incorporating 'fulvescens' with tristis.
Genetic studies have concluded that a morphotype ('appearance') is not a reliable guide to genetic composition i.e. an appearance typical of a given taxon does not reliably indicate an absence of genetic admixture.
Thus, individuals with a given morphotype and also matching mtDNA might yet harbour 'mixed' alleles (e.g. a tristis morphotype with tristis mtDNA may still harbour a proportion of abietinus alleles).
Shipilina et al. wrote : ‘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’.
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 supportive sound-recordings and sonograms were not provided). This was interpreted by van den Berg et al. as indicating extensive variation among thoroughbred tristis but the findings of Shipilina et al. (2017) proffer 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. However, this presupposes that Chiffchaffs deemed by their ringers to look like abietinus were validly diagnosed as such. In a more detailed analysis based upon a ten-year study in the Netherlands, van der Spek & de Knijff (2021) reported that, with better understanding by ringers of the requisite features of tristis, individuals subsequently found to carry tristis mtDNA were also diagnosed as tristis in the field in the vast majority of cases. This indicates that, in the earlier study, the ringers were not adequately informed to apply appropriate criteria in diagnosing tristis. While photos will not provide certainty, several published from the earlier study suggest that some Chiffchaffs diagnosed as abietinus were in fact reasonable candidates for tristis while others exhibited 'fulvescens' traits rather than suggesting abietinus. Using whoel genome analysis, the Chiffchaffs trapped during the subsequent study should provide a more suitable set for investigating the issues of hybridization and introgression.
Marova et al. (2017) noted that their genetic studies in the sympatric zone suggested that anomalous Chiffchaffs with 'misplaced' olive and yellow were 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 and even a combination of features may fail to diagnose some individuals correctly. Genetic studies indicate that both phenotype and vocalisations are determined by a relatively small part of the genome, so that compliance with the criteria for appearance and vocalisations for tristis may yet mask genetic admixture.
Studies are in progress to find significant associations between specific alleles and phenotypic traits of interest (e.g. Talla et al. 2017).
Despite the demonstrated complexities in diagnosing tristis reliably, 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.
Clearly, first generation hybrids and early backcrosses from the Urals region ('riphaeus' types with well-mixed genes) cannot be regarded as Siberian Chiffchaffs.
Pragmatically, trace levels of introgression (and slight 'fulvescens' traits sensu stricto), among individuals across the allopatric West Siberian Plain, should not disbar diagnosis as 'Siberian Chiffchaff'.
Despite significant differences in the levels of genetic mixing in the sympatric and allopatric populations, there will be individual variations 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 applied.
Key assumptions in adopting a 'pragmatic' approach are that: (a) most tristis candidates reaching western Europe originate from outside the hybrid zone (this remains to be demonstrated and, conceivably, may be false); (b) the vast majority of 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.
Hence, to validate the 'pragmatic' approach, 'whole genome sequencing' is required for a meaningful sample of tristis candidates reaching Western Europe and for 'fulvescens' individuals originating in the West Siberian Plain.
Chiffchaffs which exhibit a deficit of olive and yellow but also relatively 'grey and white' plumage (lacking the characteristic tan-brown and buff' hues of tristis) should not be assumed to be tristis adversely affected by light conditions. Careful and extended observations confirm that a 'grey and white' appearance can be genuine and enduring. Such individuals may include paler and greyer examples of abietinus, 'riphaeus' type hybrids from the region of sympatry and, conceivably, Chiffchaffs from the SE of the species' range, which combine greyer upperparts and whiter underparts with a tristis-like call.
When evaluating the status of tristis in Britain, using 2008 as a sample year, the BBRC 'tristis panel' adopted criteria based upon pragmatic yet conservative criteria. In order to exclude likely hybrids (as practicably as possible) 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. Song and call were also given due consideration but not deemed to be unequivocally diagnostic (Stoddart 2008, Dean et al. 2010).
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.