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.

• 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 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. Some Chiffchaffs diagnosed by their ringers as abietinus were in fact reasonable candidates for tristis (based upon published photos) while others exhibited 'fulvescens' traits rather than suggesting abietinus. 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. The issue of hybrid origins for anomalous individuals was still under investigation.

• Marova et al. (2017) noted that 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.

• 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.

• 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).

• 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 'Siberian Chiffchaff'. (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 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) 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.

• To validate a 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.