Taken at the Chicago Botanic Gardens in Glencoe, Illinois (USA) with a Canon 7D DSLR and 100-400mm Lens.
Red Tailed Hawk Close Up Profile. I had to google to find out more about what looks like a loose contact lens. What I found out is that many birds of prey have two or three eyelids, if you will. The half shut one you see in this shot is called a nictitating membrane.
The nictitating membrane is a transparent eye lid that closes horizontally across the eye instead of vertically like a normal eye lid. It is used to protect the eye and to help keep it moist while maintaining the birds sight.
I also managed to capture a couple shots with this membrane in the completely close position which I hope to get posted in the next day or so.