and came to the conclusion that the nearest it came to be being - not that is necessarily was as Yellow Wags are notoriously variable and races interbreed where they meet - a Green-crowned Yellow Wagtail M. - flava taivana.
The details below are from Wikipedia:
M. f. taivana (Swinhoe, 1863) – Green-crowned Yellow Wagtail or Kuril Yellow Wagtail. Like flavissima but darker above, ears much darker, almost black. Sexes similar.
Breeding: between ranges of M.f. plexa and M.f. tschutschensis S via Sakhalin to N Hokkaido.
Winter: Myanmar (Burma) to Taiwan and S to Wallacea.
I'll bet Bob's heart went pit-a-pat-a-pit, that vital organ does tend to when one finds something odd!! It also shows the need to keep one's eyes open.
And two small bits of birdy news from England. The first this morning from my sister (she even owns up to being related to me!) that the first 2 Sand Martins have turned up at a colony she watches on the East Yorkshire coast south of Bridlington and at the same time she was watching a Barn Owl hunting at 9 in the morning in broad daylight. The second was from Mike in Worcestershire who not only saw his first Swallow of the year but also had a White Wagtail in his garden, this latter a first for his garden.