Face masks are helmets to protect the user from getting hurt. If you look at them at night where the moon is just right, the helmet will glimmer and shine it's glory received from the owl who owns it.
Almost every guardian in the movie wore helmets, but in the books, it shows otherwise. It was only mentioned in the very first scene at the Great Ga' Hoole Tree in The Journey that some wore helmets. The only owl shown in the cover arts of the books wearing a helmet is, of course, Metal Beak. The only owl specifically recorded to have worn a helmet (besides Metal Beak) was Gwyndor during The War of the Ember before his death. In the series, mostly only the highest class leaders in a battle wore them. They were tremendously heavy, extremely difficult to fit, muffled an owls hearing, and couldn't stand the owls' neck's flexibility. They would only have one, only one, sight range, making owls wearing them have a huge blind spot. Only owls who were trained to use them have them and stay alive.