They're playing blind man's buff??

Hah! :) The masks are for keeping the flies away from their eyes in summer - the part over their eyes is a see-through mesh, it only looks opaque from a distance. :D

(the first time Sophie saw a horse with their summer mask on she was a little puppy, and the mask was bright purple with big silvery bug-eyes... she was all concerned, like "omg, what happened to your head???" :D )