Avian eyes have the basic structure of mammalian eyes except they lack the retractor bulbi. Unlike mammals, avian eyes contain the quadratus and pyramidalis muscles, which act on a tendon to close the nictitating membrane. This membrane may be an extended part of the pyramidalis tendon. It moistens the eye by brushing fluid across the cornea.
The extrinsic eye muscles of the bird eye are more oblique
than the mammal eye and wrap around tightly against the eye. When comparing
the ratios of eye size and muscle size, generally mammal eyes have larger
muscles than birds. The avian eye is fixed in place within a large socket,
therefore the eyes are not as mobile and the muscles are smaller. Birds
rely primarily on their necks to view their surroundings rather than eye