Red-tailed Hawk gives a menacing look. This guy (or gal) looks a bit mean, eh? It was staking out Union Reservoir in Longmont this past weekend. This capture gives a nice look at its closed, inner eyelid. Many birds have what is called a nictitating membrane. This membrane wipes front to back every few seconds, cleaning the cornea. It is translucent and the hawk can actually see through it.