White Wagtail. Kinmen island, Taiwan (1 Viewer)

Please help me to ID the subspecies of this White Wagtail. I think a little chance of baicalensis than leucopsis by the great/median coverts pattern and grey rump. If so, this could be a rarity in this area. Thanks!

Photographed on 04, Feb., 2017. Shagang ranch, Kinmen County, Taiwan. (GPS: 24.478374 N, 118.323920 E)

I would have said that this is a first-winter female leucopsis. I see similar looking birds from time to time in Hong Kong, and have also wondered about baicalensis in the past (there are currently no accepted records of baicalensis here, but it is a likely vagrant).

The two ages of tertials suggests to me that it is a first-winter (I would expect an adult to have all tertials same age). It seems quite common for leucopsis to moult only the longest tertial in first winter, although I'm not sure if that is also true of other subspecies.
I think there may be too much white in the coverts for first-winter baicalensis, but typical for leucopsis. The median coverts are very extensively white, with hardly any black showing, and the greater coverts have fairly broad white fringes.
The upperparts are fairly pale grey, but I think that this is within the variation of leucopsis, especially first-winter females. I think that the black on the rump is probably a little too extensive for baicalensis, reaching slightly higher than the second tertial and gradually fading into grey on the mantle.

If anyone has more experience of separating these two I'd be interested to hear more opinions - I'm sometimes concerned whether I am overlooking baicalensis in Hong Kong.

Thanks, johnallcock. It really help me a lot. I have spent much time for searching baicalensis in Taiwan, but still in vain. The longest tertial is actually moulting first for the first-winter leucopsis in my experience of Taiwan.