Thanks to David and John, we saw the bird right after arrival.
The final identity? I have no idea, but I don’t think this is a straightforward Motacilla flava leucocephala. The bird we saw showed whitish head (with some grey tinge) and broad wing bars, these two features seem to be good for leucocephala. However, I would expect a male leucocephala (this is a male because of the bright yellow lowerpart) should show more whitish head, chin and moustache. The nape is too dark, also showing a 'blackish lateral crown'. And further checked in computer, we found more anti-leucocephala features as below....
For the call, we tried to record when flushing the bird, showed an Eastern sonogram eventually.
From my opinion, this is not a leucocephala, or any 'pure' Western Yellow Wagtail mainly of the pro-Eastern Yellow Wagtail call (though the broad wingbar). I cannot have a solid conclusion of its ID and its better to wait for expert like Per Alstrom to give us more lights on it.
By the way, the checklist today,
nothing else interesting: https://ebird.org/view/checklist/S54299465
the Yellow Wagtail
a heavily cropped shot - you can see the very dark uppertail, dark nape, the crown is bit black too. The broad wingbars are interesting. Two tones in greater coverts, proved this is a first-summer bird?
the wingbars are very distinctive
the plumage is strange rather than matching leucocephala
as John Allcock suggested, the white feathering on tibiae is interesting too, not seen in typical Yellow Wagtail in HK. The length of hind claw can be a good feature to tell (like Richard's VS Blyth's Pipit), Eastern should show longer but hard to see or compare in this photo.
we tried to listen but really could not hear any Western call in field
my personal thought is a partial leucistic Eastern Yellow Wagtail, with uncertain subspecies (thought the greyish tone on head may rule out taivana. As seen here, there is yellow feathering on its head, should be good enough to rule out (pure) leucocephala
call of the 'white-headed' wagtail - probably still Eastern
typical Eastern Yellow Wagtail taivana molting
no lifer but anyway still an interesting lesson to learn.
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