Studies in the abietinus/tristis overlap zone conducted by Marova and colleagues
A book on Iberian Chiffchaff Phylloscopus ibericus (published in 2013, see <here>) also includes material on other members of the chiffchaff complex. Most chapters are in Spanish but with English summaries. One of the chapters, in English, is a fuller account of the studies by Irina Marova and colleagues in the abietinus/tristis overlap zone.
Among much material of interest, it is confirmed that the overlap zone extends from the southern Urals right to the Kanin Peninsula. Within this large area, the locations which host individuals with mixed song and with what the authors call ‘transitive’ features are fragmented. Their full extent remains to be determined. It may be that introgression extends east to the Ob (the authors’ opinion is that ‘fulvescens’ arises from introgression – see discussions in Appendices 1 & 2 to main text). The ‘width’ of these areas appears to be determined by habitat requirements. In the Urals, the transition zone between suitable habitats is narrow and hence the area where ‘intermediate’ birds occur is narrow. Much further north, around Arkhangelsk, the habitat transition zone is broader and the area occupied by ‘intermediates’ is correspondingly broader. The authors concluded that 24.5% of birds are hybrids in the southern Urals and as high as 60% (29 of 48 individuals examined) in the areas of co-occurrence in the Arkhangelsk region.
The authors located a narrow region in the Southern Urals which embraced a transition zone from abietinus in the west, through 'intermediate' individuals, to tristis in the east. The song of 'mixed singers' exhibited a progressively higher number of tristis-like ascending notes from the northwest to the southeast of this region. Distribution of vocal indices (the proportion of ascending notes in a song-phrase) 'corresponds to distribution of phenotypes in the whole studies area'.
Individuals with ‘intermediate’ plumage were also found to have (on average) intermediate biometrics (Fig. 15 in their chapter). In their phenotype classifications, birds with any yellow on the underparts were classified as ‘intermediate’ – only individuals entirely lacking yellow on the underparts were classified as pure tristis. Thus, the correlation of vocal index with distribution of phenotypes and the existence of biometric distinctions are very significant findings. They appear to counter the suggestion that mismatches between phenotype and haplotype (appearance and genetics) indicate simply that ‘pure’ tristis has wider plumage limits than traditionally thought. They also supports the authors’ opinion that yellow on the underparts is an indication of introgression.
The mysterious ‘third haplotype’ is again discussed. In the southern Urals, the authors found a haplotype-distribution comprising 25% abietinus, 54% tristis-1 (the ‘standard’ haplotype for tristis) and 21% tristis-2 (the ‘third haplotype’) - that is quite a high proportion. They did not find the 'third haploype' at all in eastern Siberia and concluded that it is endemic to the Urals region. Genetic studies by de Knijff et al. (2012) have found considerable haplotype variation among migrants / winter visitors in the Netherlands. Comparison of variation in mtDNA haplotype in the regions of sympatry and allopatry, respectively, may well provide further insights. Helbig et al. (1996) wrote that: '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.’ Data on haplotype variation in the Yenisey published by Helbig et al. clustered more-tightly than haplotype data from the Netherlands published by de Knijff et al.
Marova et al. also discuss briefly the increasing number of birds reaching (and wintering in) Europe. The authors discuss whether these might come from the area of expansion in European Russia (e.g. areas of co-occurrence in Arkhangelsk, where hybrids were found by this research to constitute 60% of the population) or from further east into Siberia (this matches my own reflections in British Birds, 2013, 106: 282 - 283)..
Marova, I. M., Shipelina, D. A., Fedorov, V. V. & Ivanitskii, V. V. 2013. Siberian and East European chiffchaffs: geographical distribution, morphological features, vocalization, phenomenon of mixed singing, and evidences of hybridization in sympatry zone. In: Rodríguez. N., Garcia, J. & Copete J. L. (eds). 2013. El mosquitero ibérico. Grupo Iberico de Anillamiento. Léon.
P.S. March 2017.
A paper by Shipilina et al. (2017) - available on-line <here> - has employed 'whole genome sequence data' to provide the much-needed confirmation of hybridization between tristis and abietinus in the zone of sympatry. This study also concluded that: 'genetic admixture is the force underlying trait variation'. See main text for further details.
SHIPILINA, D., SERBYN, M., IVANITSKII, V., MAROVA, I. & BACKSTRÖM, N. 2017. Patterns of genetic, phenotypic, and acoustic variation across a chiffchaff (Phylloscopus collybita abietinus/tristis) hybrid zone. Ecology and Evolution 2017; 1–12.