All birds have a second eyelid that cleans and protects their eyes. It is called the nictitating membrane. Hawks and owls have large eyes that protrude and need constant cleaning.
In these pictures you can see how the clear cornea is right out there in front. Any debris or dust blowing by will stick to this moist surface.
This is the Red Tailed Hawk that has been hunting in Springbrook's bird feeding area most of the winter. It sits in the tree right outside the windows, and I took these pictures through the windows in the last few days.
This picture shows the nictitating membrane that sweeps the eye from the side in just a milisecond. You can see how this membrane is not quite clear, and is a bit more opaque in the Barred owl below.
I took this picture also in the birdfeeding area at Springbrook. These birds watch constantly for prey and need the best vision possible. The nictitating membrane helps make that possible.
The Barred owl's eyes are large and almost a deep black when the membrane is pulled back.