The 'Siberian Chiffchaffs' at Hams Hall and Ladywalk NR 

There have been regular records of 'Siberian Chiffchaffs' (Phylloscopus collybita tristis) along the River Tame at Hams Hall and Ladywalk NR for "many" years (literature not to hand). The Edison Road outfall area is a literal hotspot in winter, with an ice/frost free zone created by warmer, treated sewage water from Minworth SF. Even in low temperatures there is an abundance of insect life to which many species are attracted. There have been up to three 'Siberian Chiffchaffs' in this area recently, following the first (near the Ladywalk entrance) on December 24th 2016. Alan Dean has provided me with a submission for the 2016 Bird Report after he saw a probable second bird at the outfall and we both agree that it is of topical interest as the bird is probably still present. Links to Alan's ongoing research into the Chiffchaff geographical intergrades, hybridisation, mixed singers etc etc are given below.

Alan Dean takes up the story ...

"On December 28th, 2016, I visited the outflow at Hams Hall, where Dave Hutton had recorded a ‘Siberian Chiffchaff’ on Dec. 26th.  The bird was soon located, frequenting the channel leading down to the river, with four Common Chiffchaffs in the same area. It displayed the following tristis-like features: crown and mantle grey-brown with a slight ‘tan’ tinge but with no visible olive; underparts with off-whitish ground-colour  with no evident yellow but a weak buffish wash on breast and flanks. It differed quite strongly from a ‘classic’ tristis, however, in that there were very prominent yellowish olive fringes to remiges and rectrices and similarly coloured streaking in the wing-coverts and lower scapulars. Faint olive in these feather tracts is within the compass of mainstream tristis but the prominence and extent of the hue on this individual placed it firmly in the ‘fulvescens’ camp, this being emphasised by a yellow wash on the fore-supercilium and upper eye-ring.

Only comprehensive genetic analysis can determine whether such strongly marked ‘fulvescens’ traits are within the range of variation of pure-bred tristis or are a result of introgression from abietinus. Currently, determining reliable nuclear DNA markers which distinguish between tristis and abietinus is proving difficult (Martin Collinson & Peter de Knijff in lit.) Mitochondrial DNA determines only the maternal lineage, so does not detect hybrids / introgression  per se. "

Addendum
Alan has added the following clarification about the Outfall bird described above:

"The current approach (pragmatic rather than genetically unequivocal) is that individuals with ‘fulvescens’ traits are acceptable pro tem as Siberian Chiffchaff as long as they have the characteristic ‘brown and buff’ hues of tristis and olive and yellow hues are sufficiently limited.  Often, ‘fulvescens’ traits are detected in digital photos but have not been observed by eye in the field. Chiffchaffs frequently move too rapidly and at a range which restricts the detail discernible by the eye. Most important of all is that in the field it is quite impossible to say that any tristis candidate does not have slight ‘fulvescens’ traits. A total absence of yellow in body plumage and eyering, for example, can only be ascertained by examination in the hand. It is not possible to be certain that such tinges are entirely absent by observation in the field. That is why – for some years -  countries such as Sweden would accept as tristis only individuals which had been trapped. Of course, this leads to a serious under-recording of the taxon, so nowadays most records committees accept individuals with limited ‘fulvescens’ traits – though, as already noted,  it’s still debated whether these traits are individual variation among pure-bred tristis or are a sign of introgression. Under this approach, accepting field identifications always implies that a bird has the characters of tristis sensu lato (i.e. classic or ‘fulvescens’ combined). We all operate with this proviso when we identify a bird as tristis. Quite a few birds seen in our region have shown ‘fulvescens’ traits in good quality digital photos – see eg www.deanar.org.uk/tristis/casestudy2.htm. It’s also the case that ‘fulvescens’ types will generally have a tristis call (and even a definite hybrid could well do so). Calls are an important guide to taxon but are not a guarantee of a pure-bred individual. There are no ‘absolutes’ as things stand – we await the outcome of more-comprehensive genetic analyses, if and when effective techniques are developed."

Alan has allowed me to publish his images on the Blog and during preparation of this today I have seen the colour variation which often exists between devices when presenting the same images. Any assessment of 'tristis' is difficult enough and reliance on photographs for identification is rarely sufficient unless they are of the very highest digital quality.

Alan's comment and images below:

"Note prominent yellowish-olive fringes to remiges, rectrices, wing-coverts and scapulars : far more evident than in 'classic' tristis. Also yellow tinge in upper eye-ring".

Alan has now accumulated a wealth of online information and the links below are highly recommended, particularly I find, as an occasional reference read. It has been regularly updated since 2009 as new information becomes available from Alan's research and his discussions with his impressive list of contacts. 

www.deanar.org.uk/tristis/tristis.htm is a link to the whole document and www.deanar.org.uk/tristis/appendices.htm#fulvescens&riphaeus  and www.deanar.org.uk/tristis/tristis.htm#fulvescens both provide further discussion about fulvescens.