Collybita, abietinus or tristis: the Chiffchaff conundrum.

Regular readers of the blog may remember that I caught a very interesting looking Chiffchaff back in September (link here). Its largely greyish-green and white livery was very different to the standard olive-green and mustard-yellow of the nominate race (collybita) Chiffchaffs that I normally catch in autumn. I thought it was likely to be a Chiffchaff of the race abietinus (the range of which includes large parts of Scandinavia, parts of eastern Europe and European Russia) but, as I commented in that earlier post, I also thought it had a look about it that potentially gave it more easterly credentials, although the early date (23/09/2018) seemed to make that unlikely.


Luckily I was able to salvage a few small feathers that it dropped in a bird bag and submitted them to Professor Martin Collinson for DNA analysis. I was certain that it would prove to be a very interesting individual whatever the result of the DNA analysis turned out to be:

1. It would be very interesting if it turned out to be collybita simply because its appearance is so unlike that of collybita.
2. It would be equally if not more interesting if it proved to be abietinus as very few have been confirmed by DNA in western Europe and also because there had been no easterly winds up to that point in September to help drift a migrant of such origin to the UK (especially the west side of the UK).
3. It would also be very interesting (if not remarkable) if it was shown to have tristis origins because of the early date, again because there had been no easterly winds up to that point in September and because its abietinus like appearance would point to it being a abietinus x tristis hybrid.

Greyish-green and white Chiffchaff caught at Billinge 23/09/2018.
Unfortunately it was not heard to call so there were no clues there. Its appearance most closely matches the descriptions for abietinus but I also knew the identification of abietinus was not necessarily that straightforward. A genetic study of migrant Chiffchaffs caught in the Netherlands found that all 23 of the birds that were identified by ringers as being abietinus, based on their appearance, turned out to have tristis mtDNA (de Knijff et al.)

The flanks were washed with pale brown similar to a tea or coffee stain on a white cloth.

The belly, breast and throat were largely white. The undertail coverts were similar to the flanks but there was a stronger buff to yellowish-buff suffusion around the vent.

The supercillium had yellow tones above and in front of the eye but was pale brown and much less distinct behind the eye. 

To say the result was eagerly awaited is a bit of an understatement and my interest was heightened after reading a paper on the genetics of Chiffchaffs caught in Britain and Ireland (Collinson et al.) that was published in British Birds earlier this year (link here). That paper looked at the genetics of 149 migrant and wintering Chiffchaffs caught during 2009 - 2017. It was interesting to see that only 9 birds in that study were found to have abietinus mtDNA and only one of those 9 was actually identified as abietinus by the ringer with the other 8 being tentatively identified as collybita or at least 'not tristis'. As for timing of autumn occurrences the few abietinus were within the same period that tristis were found with the earliest of either race being a tristis caught on the Isle of May on 28/09/2015. The study also found one Chiffchaff that was submitted as a potential tristis actually had collybita mtDNA, however, its quite dull grey appearance was considered to be unlike any of the Chiffchaff races and it was suggested that it was an aberrantly plumaged collybita.

Well I didn't have to wait too long to get the DNA result and I received an email from Thom Shannon, a PhD student working with Martin Collinson, saying that the mtDNA they recovered was that of a tristis Chiffchaff. Thom also commented that the plumage was a much better fit for an abietinus type and that they were of the opinion that it was a clear abietinus/tristis intergrade. As I said earlier the result was going to be interesting whichever way it went and to my mind the combination of its abietinus type appearance, its tristis mtDNA and the early date is the most intriguing outcome of all. 

For those of you that may not know mtDNA is only inherited through the maternal line so it only tells us that its Mum, Grandma or a direct female relative further back through the generations was a 'pure' tristis Chiffchaff. There is no way of knowing from the mtDNA or the bird's appearance if its genetic make-up is more tristis than abietinus or vice versa. Some may be tempted to think it is more abietinus than tristis because its appearance is a much better fit for abietinus but genetic studies on the breeding grounds have shown that is not necessarily the case (Shipilina et al.; Marova et al.). In those studies some birds with abietinus morphology (appearance) were found to have tristis mtDNA and a predominance of tristis DNA overall.

So the mismatch between the Billinge bird's appearance and its mtDNA points to it being an abietinus/tristis intergrade (hybrid) and one thing we can we can say with some certainty about such intergrades is where they originate. The contact zone (the area where the two races meet and overlap) is a relatively narrow band running from the southern Urals northwards to the White Sea and genetic studies have shown that past and ongoing hybridisation occurs in this area and that a similar mismatch of appearance and mtDNA is not found in other parts of either subspecies breeding range (Marova et al and Shipilina et al). Therefore, it is reasonable to conclude that this particular bird originated in that contact zone and will have travelled around 3250 km to reach Billinge.


Approximate zone of contact between abietinus and tristis (redrawn from Shipilina et al. and Marova et al.) and location of the site where the bird was caught.

There are other aspects to this record that make it particularly interesting with the early date being one of them. The Billinge bird is a week earlier than the earliest tristis identified in genetic study by Collinson et al and if you factor in the westerly location and absence of easterly winds up to that point in September it makes that early date even more exceptional. We will never know when it actually arrived in the UK but it was presumably at least a day or two before the capture date at Billinge, if not longer. In addition, it is a bird that calls into question what we know or think we know about the likelihood of abietinus occurring in western Europe and the UK in particular. I appreciate it is not safe to draw any firm conclusions from just one individual but if abietinus is the regular, if relatively scarce, passage migrant it is generally thought to be then the Billinge bird should have had a much greater chance of being found to have abietinus mtDNA as opposed to the tristis mtDNA that was found.

I am no expert on the subject but I would have thought there are far more 'pure' abietinus in this world (birds with abietinus appearance and abietinus mtDNA) than there are abietinus looking hybrids that have tristis mtDNA, especially when you consider the large range that abietinus occupies west of the overlap zone and the relatively small area of overlap zone itself, where all abietinus x tristis intergrades are thought to originate. It has been suggested, by some who know far more about this subject than me, that many abietinus are so similar to collybita that they probably go unnoticed in western Europe but even if that is the case I would have still thought that should leave enough abietinus that look sufficiently different to be picked out with relative ease, at least in the hand if not in the field. More importantly I would have also thought it more likely that those abietinus that can be identified from their appearance should still outnumber abietinus/tristis intergrades with abietinus morphology, assuming they are the regular and more common passage migrant that presumed wisdom suggests.

As I said earlier it is not safe to draw any conclusions from one individual but the Billinge bird is another record that adds to the uncertainty that surrounds the true status of abietinus as a migrant to the UK. In addition it extends the period when birds of tristis origin can occur and, if nothing else, is simply a very interesting record in its own right as we can be reasonably certain of the region it came from.

References:
Collinson, J.M., Murcia, A., Ladeira, G., Dewars, K., Roberts, F. & Shannon, T. 2018. Siberian and Scandinavian Common Chiffchaffs in Britain and Ireland - a genetic study. British Birds 111: 384-394.

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.

Marova, I.M.,Shipilina, D., Federov, V., Alekseev, V. & Ivanitskii, V. 2017. Interaction between Common and Siberian Chiffchaff in a contact zone. Ornis Fennica 94: 66-81.

de Knijff, P., van der Spek, V. & Fisher, J. 2012. Genetic identity of grey chiffchaffs trapped in the Netherlands in autumns of 2009 - 2011. Dutch Birding 34: 386-392.