The original description of “Phylloscopus tristis” (Blyth 1843, Journ. Asiatic Soc. Bengal XII, p. 966-7) was based upon individuals on the wintering grounds in India and is as follows:
'Ph. tristis, Nobis. Closely allied to Ph. rufus, but devoid of any greenish or yellowish tinge on the plumage, except on the fore-part of the wing underneath, and faintly margining the quills and tail externally; legs and claws black, or rather dull black (much darker than in Ph. rufus), except the under-surface of the toes which is yellow; bill also blackish, tinged with yellow at base of lower mandible, and the gape also yellow. Length four inches and a half, by six inches and a half across; of wing two inches and one-eighth; tail an inch and three-quarters; bill to gape half an inch; and tarsus seven-sixteenths of an inch. General colour greyish-brown, beneath paler and albescent, a faint rufous tinge on the breast, and not trace of yellowish on the lower tail-coverts, nor elsewhere than as described.*
[Footnote] * There is the faintest possible greenish tinge on the upper-parts of some that I have since procured, which colour is most developed on the margins of the secondaries, towards their base.'
[Ed note: Ph. rufus was the scientific name for collybita and abietinus combined, before their separation.]
Individuals wintering in India may not all originate from eastern Siberia but the 'type description' conforms with modern interpretations of 'classic' tristis, viz. plumage lacking yellow away from the underwing and lacking olive except faintly on the margins of remiges and rectrices; upperparts grey-brown, underparts off-white with a faint 'rufous' tinge on the breast.
The form 'fulvescens' was first described as a new taxon by Severtzov (1873), based upon a series of 100 migrants collected in Turkestan.
Severtzov’s description (kindly provided and translated by Dr Vladimir Loskot, Curator of the Ornithological Department, Zoological Institute, Russian Academy of Sciences, St. Petersburg) was as follows:
‘Ficedula (Phyllopneuste) fulvescens, nob. - (Ph. tristis? Gould). Upper parts, from forehead to upper tail coverts and small wing coverts rusty-grey with olive tint, olive-brown in autumn; supercilium and under parts rather pale rufous-yellowish, brighter in autumn; cheeks not pure rusty; wings and tail feathers blackish, with olive fringes which, on coverts, cover blackish middle of feathers; small underwing coverts sulphur yellow; first primary twice as long as its coverts; 3=4=5>6>7>2>8. Male and female do not differ from each other; in juvenile birds, differing in general only in looser texture of feathers, sometimes unclear longitudinal stripes of pale sulphur yellow colour are present on the breast; in others, the breast stripes are similar but greyish (var. naevia), and then all the plumage is more greyish than usually. Bill and legs black; bill is relatively small even for a Leaf Warbler, and claws are large, especially on the hind and mid toes.’
Colour nomenclature is always problematic, especially in translation, but this description does equate with that of tristis with additional traces of olive and yellow. Note that in references to 'fulvescens' traits occurring east of the Yenisey, Ticehurst (1938) expressly includes olive on the rump and wing-margins as 'fulvescens' traits, as mentioned by Sushkin (1925), but does not explicitly mention 'misplaced' yellow (see Appendix 3).
In the region of overlap between tristis and abietinus north and west from the southern Urals, Chiffchaffs are encountered with 'intermediate' appearance and employing ‘mixed' song. They have been studied in depth by Marova & Leonovich (1993), Marova & Alexseev (2008), Lindholm (2008), Marova et al (2013) and Shipilina et al. (2017). In true 'mixed singers', each song phrase comprises an intimate mixture of notes from the songs of tristis and abietinus.The precise extent of the zone or zones of overlap and ‘mixed singing’ are currently undetermined. Marova & Leonovich (1993) mapped an outline embracing a region some 1500km long by 400 km wide but, within this large area, the co-occurrence of the two forms is fragmented (Marova & Alekseev 2008). All these studies concluded that a combination of 'mixed singing' and intermediate plumage indicated a significant level of hybridization.
Using a combination of morphological, vocal and DNA data, Marova et al (2009 and 2013) and Shipilina et al. (2017) concluded that hybridization is frequent in the geographical zone of overlap. Earlier studies examined only mtDNA, which is inherited solely from the maternal side, so does not provide unequivocal proof of hybrid origin. The study by Shipilina et al. involved novel techniques to examine 'whole genome sequence data'. This study confirmed that chiffchaffs within the sympatric region have 'a mix of genetic ancestry indicating extensive ongoing and past gene flow'. The paper indicated that ‘genetic admixture is the driving force underlying trait variation in the sympatric region’ and that intermediate-looking individuals are hybrids. It infers that even traces of misplaced yellow found in individuals in the West Siberian Plain, east of the sympatric region, are a result of ‘fractional’ introgression of abietinus alleles.
Two earlier studies, by de Knijff et al. 2012 (Dutch Birding 34: 386-392, viewable on-line <here>), and by Greg Conway & Staffan Bensch (unpublished), had already prompted much discussion. Both indicated that, in late autumn and winter, Chiffchaffs which did not match nominate collybita in appearance consistently carried an mtDNA haplotype of tristis. In de Knijff et al.'s study, many of the 30 individuals deemed paler and less-colourful than nominate collybita had been tentatively assigned to abietinus by their ringers. Yet genetic analysis detected no abietinus mtDNA haplotypes at all. These findings had several ramifications. Firstly, it was inferred that the plumage of pure-bred tristis is much wider than traditionally believed. Secondly, it was suggested that all Chiffchaffs which are paler and less-colourful than nominate collybita are tristis (and that abietinus is always very similar to collybita). It should be noted here that these two presumptions inferred a very wide variability among tristis and yet, in contrast, a very limited variability among abietinus (which is arguably counter-intuitive).
As a result of these studies, it was also questioned whether abietinus reaches the UK at all, at least in late autumn and winter. De Knijff et al. pre-selected their Chiffchaffs on the basis of non-collybita-like appearance and could, therefore, have missed abietinus (many of which appear similar to collybita). However, in their study, Conway and Bensch did not preselect Chiffchaffs on the basis of appearance, yet a dearth of abietinus was found here too. Although indications remain that they are uncommon (significantly scarcer than tristis for example), it has now been confirmed that abietinus does occur in late autumn and winter. For example, Chiffchaffs with abietinus mtDNA have reached Scotland in November and Ireland in January - per Martin Collinson. The status of the form earlier in the autumn remains to be fully clarified.
In their study, de Knijff et al. referred to their paler and less-colourful Chiffchaffs as 'grey' but this has been over-interpreted, as none in the published photos matches the morphology of 'grey and white Bonelli's-like' Chiffchaffs as discussed by Dean & Svensson 2005 (see main text, including plate 5 et seq.). Currently, there is little or no published data on the genetic identity of such truly 'Bonelli's-like' individuals. Judging from the photos published by de Knijff et al., a significant proportion of their Chiffchaffs possessed evident tristis-like 'tan-brown and buff' hues. This applies to the individuals in plates 531 and 533, for example. These two appear very similar in the published images, yet the former was identified by its ringers as abietinus while the latter was identified by its ringers as tristis. Thus, some of the individuals diagnosed by their ringers as abietinus were, in fact, reasonable candidates for tristis based upon field appearance and this needs to be kept in mind when discussing the genetic findings of the paper. However, several individuals in the plates do lack dominant tristis traits and have evident 'additional' olive and yellow. The individual in plate 530 in the paper has quite prominent olive hues over its entire upperparts (and is not at all 'grey'). That individual was reasonably anticipated to be abietinus yet possessed tristis mtDNA.
As was noted subsequently by Collinson et al. (2013), there were two possible interpretations of de Knijff et al.'s findings (keeping the proviso over the ringers' attributions in mind) : either the morphology of pure-bred tristis is wider than traditionally believed or there is on-going introgression of tristis mtDNA into abietinus populations. The results of 'whole genome' sequence analysis by Shipilina et al. has confirmed that it is the latter.
Of comparable importance is the implication that, whatever its true status, prevailing ideas about the appearance and plumage limits of abietinus are just as fickle as those relating to tristis (at least among observers residing outside the breeding range of abietinus).
Even among those who have acknowledged the probability that tristis and abietinus hybridize, some have suggested that, as the region of sympatry between tristis and abietinus is narrow in comparison with the total range of tristis, the proportion of individuals reaching western Europe from the region of potential hybridization will be insignificant and can be ignored. However, this assumes an equal likelihood of arrival from all parts of the range. The increasing number of Siberian Chiffchaffs reported in western Europe may well reflect an inherited migration strategy originating in a relatively restricted part of the total range of tristis. If this area is situated in the west of the range and embraces the region of sympatry and range expansion of tristis, then the proportion of 'Siberian Chiffchaffs' reaching the UK and other areas of western Europe from the zone of potential hybridization should not be underestimated (Dean 2013 and Marova et al 2013). Individuals with 'fulvescens' traits (sensu stricto) are relatively common among tristis candidates reaching western Europe. Shipilina et al. concluded that even limited 'fulvescens' traits found among Chiffchaffs beyond teh sympatric zone, acrosss the West Siberian Plain, were nevertheless a product of 'fractional introgression'. Some tristis candidates are overwhelmingly tristis-like in the field, with very slight 'fulvescens' traits revealed only in the hand or in the best, colour-accurate digital photos (and colour accuracy can be difficult to ascertain). How best to deal with such individuals is discussed in the main text.
Unexpectedly, the genetic studies by Marova et al. (see <here>) and independent studies by Greg Conway and Staffan Bensch (in lit) both found a third mtDNA haplotype among the Chiffchaffs in their samples. While closer to tristis than abietinus, this third haplotype differed in a significant marker from the then recognised haplotypes of both tristis and abietinus. Marova et al. indicated that this third haplotype was restricted to the area of sympatry in the Urals region and they did not find it among tristis in the Yenisey valley.
In their study, de Knijff et al. (2012) illustrated wide variation in tristis haplotypes among tristis types trapped in the Netherlands (see their Fig. 1). I do not have the genetic knowledge to know whether the 'haplotype variations' described in various studies are directly comparable. However, some non-avian genetic studies (e.g. <here>) have reported variation between the mtDNA genome sequences of hybrids and that of their female parents (a significantly higher number of base substitutions), while Helbig et al. (1996) wrote: ‘Inheritance of mitochondrial mtDNA is clonal (no recombination), which means that events of fertile hybridization, even if they occur rarely or have occurred in the relatively distant past, may potentially be reflected in the mitochondrial genetic make-up of a species.’. However, among ten tristis from the Yensisey analyzed by Helbig et al., only one haplotype differed by more than two substitutions compared with the commonest haplotype. In contrast, in the study by de Knijff et al., of 30 Chiffchaffs diagnosed with tristis mtDNA trapped in the Netherlands, 12 differed by three or more substitutions compared with the commonest haplotype (see Fig. 1 in de Knijff et al., 2012). Thus, it seems the Netherlands samples clustered far less tightly than did those from the Yenisey.
The characters of Chiffchaffs in the sympatric region (around the Urals) and of Chiffchaffs across the allopatric West Siberian Plain have frequently been conflated under the name 'fulvescens'. Yet Ticehurst (1938) regarded 'fulvescens' from western Siberia as a variant of tristis (with brighter olive on rump and wing edges, as per the treatment of Sushkin 1925 and others) and separately diagnosed as hybrids birds with more-intermediate appearance from the sympatric region around the Urals (and 'on migration' elsewhere - see 'Explanatory note' below ). He referred to the historical designation of such hybrids as 'riphaeus', though he did not advocate the use of 'formal' names which lacked rigorous taxonomic significance. Such individuals were: 'greyer above than is usual with abietinus but still with too much green in the upperparts for tristis and a trace of yellow on the breast which tristis never has; the amount of yellow in the supercilium and eye-rim is reduced'. Thus, Ticehurst recognised a division between the two groups : 'fulvescens' from western Siberia (differing from classic tristis by enhanced olive in rump and wing edges) and 'riphaeus' from the sympatric region around the Urals (intermediate between abietinus and tristis and deemed to be hybrids). Similarly, some Russian authors (e.g. Buturlin & Dementiev 1937) considered 'riphaeus' from the Urals region as a product of hybridization between fulvescens (sensu Sushkin 1925) and abietinus. Unfortunately, the distinction between the sympatric and allopatric regions has subsequently become blurred. Several authors (including Marova et al., 1993 et seq.) have employed the term 'fulvescens' in reference to any Chiffchaff of basically tristis-like livery but with 'additional' yellow compared with 'classic' individuals. Thus, unlike Ticehurst, they employ the term for individuals with any degree of 'additional' yellow, whether originating from the sympatric zone or from the allopatric zone. While it may be that the differences from 'classic' tristis in each case arise from introgression, the levels of 'genetic mixing' in the two regions (sympatric and allopatric) are on significantly different scales. The extent and nature of character variation across the sympatric and allopatric regions, respectively, is in need of further clarification while the levels of genetic admixture in the two regions are certainly significantly different. Such issues have relevance to decisions on what does and does not represent a 'diagnosable Siberian Chiffchaff'.
Thus, although not applied with taxonomic significance, the terms 'fulvescens' and 'riphaeus' used colloquially, and their distinction as described above, remain helpful in discussions of the issue of hybridization and introgression. The potential for gene flow in a zone of sympatry ('riphaeus' zone) is clearly high while in a region of allopatry ('fulvescens' zone sensu stricto) it is relatively low. This significant difference in levels of gene flow can be pertinent to how individuals from the two regions are designated. In this text, the distinction recognised by Ticehurst is maintained and 'fulvescens' is used sensu stricto to refer to the Chiffchaffs from the allopatric West Siberian Plain which match 'classic' tristis very closely apart from slight traces of 'misplaced' yellow and olive (often not detectable in the field). The term 'riphaeus' is applied colloquially to the birds with more-evidently intermediate appearance found in the sympatric zone, from the southern Urals north to the Kanin peninsula).
To indicate their colloquial usage, the names should appear between inverted commas. The issue of appropriate identification criteria is examined in the main text
Martens & Meincke (1989) reported that the song of ‘fulvescens’ was consistent with that of tristis across its entire range but their studies did not extend west of the Urals. Thus, when Marova & Leonovich (1993) reported ‘mixed singing’ among birds which they termed ‘fulvescens’, their results seemed in conflict with those of Martens & Meincke (1989). However, the two study areas were on opposite sides of the Urals and hence geographically distinct. This is an example of how confusion can arise when the name ‘fulvescens’ is applied in these two rather different contexts (areas of sympatry and allopatry, respectively) and, in their discussion of the conclusions of Marova & Leonovich versus those of Martens & Meincke, this ambiguity was not adequately clarified by Dean & Svensson (2005).
[Explanatory note. Locations noted by Ticehurst (1938) of individuals with intermediate, 'riphaeus' traits (and deemed to be 'on passage', presumably based upon date obtained) included the south coast of the Caspian and the Elborz Mountains, in Iran. Vaurie (1954) included the birds breeding in this region with 'fulvescens', despite their disjunct distribution, and this was repeated by Williamson (1962), though he subsumed 'fulvescens' with tristis. (Neither Vaurie nor Williamson addressed the distinction recognised by Ticehurst between the terms 'riphaeus' and 'fulvescens'). Chiffchaffs breeding in woodlands in NE Iran and Kopet Dagh in Turkmenistan are indeed greyer above, whiter below and deficient in yellow compared with typical abietinus - but are now designated as menzbieri. At the time of Ticehurst's monograph (1938), menzbieri had only recently been described (Shestoperov 1937) and was not included (while caucasicus was not described (by Loskot) until 1991). As late as 1992, this confusion over menzbieri persisted, with BWP6 opting to include it within tristis but noting that 'it may belong to abietinus'. The precise appearance of menzbieri remains to be defined unequivocally. See <here> for examples of Chiffchaffs in the Elborz mountains of Iran and <below> for further comments on the calls and appearance of certain Chiffchaffs in the Middle East.]