I took these photos of a non-breeding White Wagtail at Mae Hia Agricultural College on September 25, 2013. Generally, it looks similar to post-breeding ‘M. a. leucopsis’ which is the commonest subspecies in Thailand, but it clearly showed some black on the cheek, which seems to be a trait of ‘M. a. alboides’, another subspecies less regularly found in Thailand.
As you can see, it has quite an extensive amount of black on its breast. Seems good for typical non-breeding ‘alboides’. However, the head pattern is just not right for neither for ‘alboides’ nor ‘leucopsis’. Instead of having fully black head with white mask, it has instead largely white head but with some black on the cheeks and what seem to be faint malar stripe.
It was enjoying a variety of meals. Here’s a small cricket.
And here’s a worm.
So in my opinion, what seems to be the closest choice is a hybrid between ‘alboides’ and ‘leucopsis’. Not only because it seems to show characteristics of both subspecies, but also because ‘leucopsis’ is the only subspecies with white face that has an overlapped breeding range with ‘alboides’. Hybrids between these two have also been reported before like this one from China. Below you can see the map showing the breeding range of each White Wagtail subspecies from all over the world.