Barred Owls have a very visible nictitating membrane, and it appears as a blue film sliding over the eye from a distance. It is actually a
third eyelid that moves diagonally across the eye from the upper corner. The eyelid is a translucent, protective covering that the owls
use when flying through thick woods and feeding young to prevent sticks or overly eager young from scratching the eye.

http://www.raptorrehab.org/raptors/barred.htm