Is this one?
Spurn has had its fair share of fine ‘obvious’ male Blue-headed Wagtails in the latter half of April 2011, with at least 4 males seen plus a male ‘Channel Wagtail’. This bird found in the evening on 23rd April was a little trickier to identify- unsurprising with flava wagtails. Clocked by warden Paul Collins at ‘Wire Dump’ as a somewhat ‘blue-headed’ looking in the field, the bird was coaxed into the Heligoland trap. Aging flava wagtails in spring is clearly tricky. We think this is a first summer male. The 3 outermost greater coverts were older with worn thin whitish fringes and perhaps more importantly the wings appeared rather more brownish than fresher blackish of adult males. Most interesting the crown was obviously peppered with blueish feathers most concentrated on the forehead. The underparts also had rich yellow emerging right throughout though was more obvious in the belly. Some young male flavissima in spring, can look more female-like, but you can see that vivid yellow is coming through in the breast and throat. This bird seems to be a slow developing 1st summer male Blue-headed Wagtail.
Photos below taken in low evening light and blue on crown and yellow in underparts could be visible/ less visible depending on light /angle. See the ‘Dover Wagtail’ at the bottom!
This bird appeared the next day in Clubley’s field with c 40 Yellow Wagtails. It’s basically a male Yellow Wagtail ssp. flavissima, but with the supercilium changing from yellow to white at the end (on both sides) and an area of pale blueish-grey on the rear crown, upper nape. Perhaps a 2nd generation ‘Channel Wagtail’ X Yellow? (hence tongue-in- cheek ‘Dover Wagtail’).
Thanks to Nathan Pickering for searching through the flava flocks and Martin Standley for the middle pic below.