Common Chiffchaff (Phylloscopus collybita) is a leaf-warbler familiar to birdwatchers in Britain, as a common summer visitor, and in much of the country as a winter visitor in small numbers. Its eastern cousin tristis, Siberian Chiffchaff, also regularly winters in Britain, and there’s currently one in Boiling Wells Lane, at the north end of St Werburghs. The stream from which the lane gets its name, and a nearby dung-heap, are attracting plenty of insects, keeping our Siberian visitor loyal to a very small area. Here’s a photo:
A lot of confusion surrounds the status of Siberian Chiffchaff in Britain, more of which below, but they are pretty distinctive birds, especially if you have the opportunity to compare one with Common Chiffchaffs, of which there are up to four at Boiling Wells currently. Siberians are paler birds, particularly on their underparts, which are whitish, lacking the yellowish-buff colours present on Common Chiffchaff. The upperparts are pale grey-brown, usually without the green tones of Common Chiffchaff, although the flight-feathers and the tail can be finely edged with green. Usually there is a warm buff colour on the cheeks and breast-sides, and there can often be a narrow pale wingbar. The bill and legs are black and so are conspicuously contrasting compared with the pale plumage. Finally, both songs and calls are different: the St Werburghs bird gives the typical call of Siberian Chiffchaff, a soft single-syllabled ‘peep’, neither rising nor falling, different from the rising, slightly disyllabic ‘hueet’ call of Common Chiffchaff. If it’s a male, it may start singing later in the winter: instead of the repetitive ‘chiff … chaff’ song, Siberian’s song has a more varied structure to it.
The confusion I mentioned above has a long history. First, in the 1950s and 1960s, several SIberian Chiffchaffs were mistaken for Greenish Warblers (Phylloscopus trochiloides), something that seems hard to believe today, with improved knowledge and better optical equipment. More recently, there has been a wide-ranging debate about the identity of non-calling birds, especially those which differ from the typical appearance described above. Midlands birder Alan Dean has highlighted several such birds which he terms “grey-and-white chiffchaffs”, and believes that these are not Siberian Chiffchaffs, but either eastern Common Chiffchaffs or the result of hybridisation where the two meet. This has caused many birders to doubt the identity of many of the birds they’ve seen.
Thankfully, things may not be as bad as Alan suggests. Birders based in Poole, Dorset have been studying their wintering chiffchaffs, and concluded that at least some of the birds which do not conform to the typical appearance still call and sing like Siberians. More recently, ornithologists in the Netherlands looked at the DNA of birds trapped for ringing, and discovered that not only did birds identified confidently as Siberian Chiffchaffs have the right DNA, so did a large number of other birds, all of which had been tentatively identified by their ringers as Scandinavian Common Chiffchaffs (Scandinavia is one area the “grey-and-white” birds have been thought to come from). Interestingly, none of the birds in this study had DNA which matched birds from Scandinavia, even though the Dutch ringers had specifically been looking out for them, and all of the normal-looking Common Chiffchaffs trapped were just that.
There has also been a study in western Russia, looking at if and how frequently the two hybridise. The answer? Yes, just like us humans, some chiffchaffs are confused about chiffchaffs and hybridisation does occur. However, it only occurs in a narrow zone some tens of miles wide where the two populations overlap. West of this for several thousands of miles are normal Common Chiffchaffs, while normal Siberian Chiffchaffs are found east almost all the way to the Pacific Ocean. Numerically, hybrids are so rare that they can’t really be the cause of Alan Dean’s puzzle.
There are dozens of other Phylloscopus warblers in Europe and Asia, and many of these are difficult to tell apart visually, but have distinctive calls and/or songs – indeed, several new species have been described based on this in the last twenty years or so. Siberian and Common Chiffchaffs seem to be another one of these species-pairs. Historically, Siberian Chiffchaff has been considered a geographical race of Common Chiffchaff, however given what we now know this is not really appropriate. Because of the narrow hybridisation zone, they are probably best considered “semispecies” (biological jargon which means that, although not everything’s completely sorted out in the overlap zone, evolution has basically done its job and the two are on separate future trajectories).
I did a little bit of birding PR today with the owners of the dung-heap: they had wondered if all the people with cameras who had been visiting this week were from Bristol City Council and were eyeing up their land for house building. I showed the bird to them and explained why it was attracting interest, and I think they’re now happy that their visitors are all just perfectly normal people who just happen to like looking at birds on dung-heaps. They regularly turn the dung, exposing more of the chiffchaff’s food, so hopefully it will hang around for quite a while.
Here’s a map showing the area the bird frequents: