Guest post by Dan Weisz
Remember that pair of red-tailed hawks from last week along River Road? Here are two more photos from that morning, each showing one hawk and it’s nictitating membrane. The nictitating membrane is a third eyelid that is translucent or clear. It is used as an extra level of safety for the eye, moistens the eye while maintaining vision, and allows the bird to still see while affording its eyes an additional amount of protection.
The nictitating membrane on the bird on the right is only halfway across her eye. this membrane always moves horizontally, unlike the “regular” eyelids which move up and down. I caught this bird mid-blink.
Nictitating comes from the latin nictare, meaning to blink. In the photo below, I’m not really certain we are looking at a nictitating membrane or a closed eyelid. The lid doesn’t look like the blue-ish translucent in photos of other birds. Either way, it’s interesting to see the prairie falcon mid-blink (taken near Rio Vista Park).
Here is a yellow-headed blackbird from Red Rock mid-blink. The lid is moving from front to back.
Also at Red Rock from last spring, this Chihuahan Raven is mid-blink. His eyelid is also moving from front to black. How can I tell this is a Chihuahuan Raven? See the white base of his neck feathers? In Chihuahuan Ravnes, they have that white base unlike Common Ravens. Their scientific name is Corvus Cryptoleucas, meaning "raven with the hidden white".
And at full nictitating state.
This broad-billed hummingbird seems to have his eyelid closed rather than his nictitating membrane. Agreed?
And at the Desert Museum last spring, we were training a new Harris’s Hawk to join the family. He often landed on the audience’s handrail, which would not have worked out so well during a typical demo. In any case, he had his nictitating membrane beginning to close in this shot.
Another Harris’s Hawk during a demo with his eye fully covered by the nictitating membrane. Raptors will use this membrane when attacking prey as an added protection. Additionally, it is also serves to protect the eyes of a parent from their chicks while they are feeding them. Nobody wants their offspring to poke them in the eye. Peregrine falcons will use the membrane repeatedly during their high-diving stoops to clear dust and debris and to moisturize their eye during the dive.