HI Jeffrey et al.:
I have found this article of use for IDing White Wagtail subspecies:

https://www.westernfieldornithologists.org/archive/V29/29(3)%20p0180-p0198.pdf

sincerely
--

Ian Paulsen
Bainbridge Island, WA, USA
Visit my BIRDBOOKER REPORT blog here:
https://birdbookerreport.blogspot.com/