I feel bad about completely pirating a birdforum thread, but it would be good to get some discussion on here about this bird.
One of the observers describes a “rasping” call, which certainly sets alarm bells ringing. For reference, I’ve attached a recording of Eastern Yellow Wagtails I made last summer in Chukotka, Russia (includes alarm and standard flight calls, sorry for the poor quality).
If the Cornwall bird is a genuine match, I’d say it is potentially the most interesting bird in the UK at the moment. I'm not sure whether the status of Eastern Yellow Wagtial (M. (f.) tschutschensis) is under review with the BOU, but it certainly should be. The AOU treat it as a full species, and it could also qualify under the BOURC TSC's criteria. I know there are a number of previous claims, but I’m not sure how many are well documented with sound recordings etc..
Several phylogenies have been published, with universal agreement that the western and eastern clades of Yellow Wagtail are highly divergent (probably not even each other's closest relatives, with citreola somewhere in the middle). Differentiation by plumage/morphology needs further study, but juvenile Easterns are highly distinctive, lacking yellow tones and being more heavily marked on the head and wings. Adults also differ in that they lack any clear sexual dimorphism, at least in the two most widespread forms, nominate tschutschensis and macronyx. By far the biggest differences, however, are in vocalisations. All eastern forms have highly distinctive calls – check out the recording – they are even more distinctive than citreola. Males of tschutschensis also have a more complex song than western forms of flava.
The taxonomic situation is muddled by a lack of data from the contact zone between eastern and western clades, which lies somewhere in central Russia (between nominante tshutschenis and thunbergi). It would be great to hear from anyone who has experience in this area…
Anyway, the Cornwall bird - is it an Eastern Yellow Wagtail? Certainly the grey plumage tones and large bill are good pointers, but plumage and morphological differences are variable and poorly understood (particularly in winter), and there may be complete overlap with western forms. It would be great if someone could get some good quality recordings of it – the calls may well be diagnostic. No doubt the biggest issue will be eliminating the south-eastern forms of “Western” Yellow Wag, which also have raspy calls, more similar to citreola.
Hopefully if this bird stays around it will galvanize some more interest in this 'form'…