An interesting Chiffchaff at Spurn

I stayed at Spurn from 11th to 13th October with a regular group of friends, formerly from Leeds, namely; Roger Barnes, Vaughan Lister, Paul Doherty, Mick Cunningham, Richard Hart and Martin Quinlan whilst James Lowen also a friend originating from Leeds was with another group of birders from Norfolk. 
There were still several Yellow-browed Warblers around following the influx on Sunday 8th October but, other than an Olive-backed Pipit found on the 11th, few new birds turned up during my stay which was characterised by gale force warm, southerly or south-westerly winds.
On the 12th a tristis type Chiffchaff was seen and photographed at the Crown & Anchor late afternoon and I spent much of the following day looking for, and then watching it. This blog post deals with its identification.
A force 6 southerly wind blew throughout the day but one of the few areas that offered some shelter was the band of trees in the car park closest to the Crown & Anchor. The tristis type Chiffchaff was reported again here early on and when I arrived at about 9:00 a Yellow-browed Warbler was seen followed by brief views of a rather grey Chiffchaff.
Fortunately the Chiffchaff appeared to be on a circuit and every 30 minutes or so reappeared at the most westerly end of these bushes.
There were several birders present and it was interesting from the start how the appearance of the bird varied to different observers with some immediately dismissing it as nominate collybita. Of those who had seen the tristis Chiffchaff early on some were of the view that this was a different bird. As the Chiffchaff did a circuit and reappeared some considered that there was more than one bird involved including a typical nominate collybita and a paler bird!
The day started off heavily overcast but occasionally the sun shone through and this certainly altered the perception of the birds colouration as did the extent to which it was in shadow in the foliage.
To my eyes the Chiffchaff always looked greyish on the crown and mantle and especially so on the nape. Whilst the edges to the remiges and retrices were fringed olive green. The underparts were pale buff with a yellow tinge to the breast sides and undertail. The supercilium was prominent infront of the eye and also tinged yellow as was the upper eyelid. The legs were black as was the bill but with slightly paler sides at its base visible in the photographs but I thought it looked entirely black in the field.
Several observers commented that the overall impression of grey above and pale below with greenish fringes to the wings and tail was slightly reminiscent of the Bonelli's Warblers. 
The bird called repeatedly at times giving a slightly disyllabic hoo-eet which to my ears was fairly typical of nominate collybita. The noise of the wind in the trees put me off attempting to record the call although in hindsight any recording is better than none!
Photographs
I took photographs when ever the bird appeared during a couple of hours in the morning and a further hour in the late afternoon. Most of the time it was overcast but there was some sunshine when photo 4 was taken.

My camera, a Canon DSLR, is set on neutral so no sharpening or colour toning changes are made when the photo is captured. White balance is left on auto. On the computer I've made slight adjustment to the overall brightness of the image and sharpened them slightly but haven't altered clarity, vibrance or saturation. These images should be as close to how the bird appeared as Canon can make it.

Photo 1 

Photo 2 taken in shadow the bird looked darker above

Photo 3 the breast has some fine yellow streaks when seen close

Photo 4 taken with some sunshine the mantle looked slightly more olive and the fringes to wings and tail are brighter

Discussion
I posted Photo 1 on Twitter and got some helpful comments which directed me to a recent British Birds article (Vol 111 July 2018 pages 389-394) and other web resources which have addressed the identification of the races of Chiffchaff with particular reference to the use of DNA from trapped birds. The excellent and detailed summary by Alan Dean on behalf of the BB tristis panel is probably the best starting point for anyone with any interest in the racial identification of Chiffchaffs. Mick Cunningham also put me on to Vincent van der Spek's blog 'Turnstones' which has some interesting information on Chiffchaff ID from ringing studies in the Netherlands.
My initial thoughts were that the presence of yellow on the supercilium, breast sides and undertail plus the lack of chestnut/ brown on the ear coverts excluded tristis and that this bird was more likely therefore to be abietinus was a somewhat over simplified view given the latest published findings.
The appearance of abietinus is now thought to be much closer to collybita and there are few verified (by DNA) records in the UK. This is odd given that the range of abietinus is closer to the UK than tristis and one might therefore expect more records, unless their appearance is so close to collybita that they are overlooked. 
A 2012 paper in Dutch Birding (DB 34 (6) 386-392) provides results of mtDNA analysis of 41 birds trapped in the autumn of 2009-2011 which did not look like typical collybita (23 were identified as abietinus by the ringers handling the birds) or from very large birds, suggestive of abietinus. Of the 41, 11 were nominate collybita and 30 were tristis and no abietinus were identified in the group. 
At the Western edge of the range of tristis birds may show some yellow, particularly in the supercilium and these birds have been referred to as a separate race fulvescens. However, a 2017 publication by Shipilina (Patterns of genetic, phenotypic, and acoustic variation across a chiffchaff (Phylloscopus collybita abietinus/tristis) hybrid zone) concluded that genetic mixing of tristis and abietinus where the races are in contact is the source of plumage variation (chiefly occurrence of 'misplaced' yellow in the plumage) in tristis.
Conclusions
1. I know a lot more about racial variation in Chiffchaff and the identification of its subspecies than I did when I took the photos!
2. There was I think only one tristis type Chiffchaff in the area at the time and the appearance of the bird varied with the light and the extent to which it was shadowed by foliage. As demonstrated by the fact that birders stood close to each other had different pereceptions of the birds appearance.
3. The bird was probably an example of tristis/abientinus interbreeding often referred to as 'fulvescens'. These are birds from the western edge of the tristis range and are generally grouped with tristis and termed as Siberian type Chiffchaff and/or western tristis.
4. The call, to my ears at least, was typical for nominate collybita. Not sure how this fits in and I perhaps need to look in to call variation in tristis with the 'misplaced' yellow. 
5. A better starting point in the search for abientinus would be slightly larger nominate collybita with perhaps slightly greyer upperparts and less yellow underparts rather than odd tristis However, it appears that many abietinus may be inseparable from collybita in the field.