Why do birds need a third eyelid?
A bird’s eyes are exposed to a lot.
Wind. Dust. Branches. Water. Fast flight. Struggling prey. Bright sun.
The nictitating membrane gives the eye another layer of protection without always blocking vision as completely as a thick outer eyelid would.
In many species it is translucent or partly transparent, so the bird may still see something while the membrane is across the eye.
What is the nictitating membrane?
It is a thin third eyelid found closest to the eyeball.
Instead of moving mainly up and down like our eyelids, it sweeps sideways across much of the eye. You can occasionally catch it in a photograph and think the bird’s eye has suddenly turned cloudy or pale.
It has not. You may simply have frozen the membrane in mid-blink.
Do birds blink?
Yes.
Bird blinking does not always look like ours. Upper and lower eyelids can move differently by species, and the third eyelid adds another kind of blink.
That is why a close-up video of a bird can look odd for a split second. The eye may seem to flash white, blue-grey, or translucent as the nictitating membrane passes over it.
Which eyelid do birds use when they sleep?
Birds generally close their outer eyelids during ordinary sleep.
Some birds can also sleep with one brain hemisphere more deeply asleep than the other. During that kind of unihemispheric sleep, one eye may remain open while the other is closed.
That lets the bird keep some watch on its surroundings.
Our where do birds sleep at night? page covers roosting and nighttime behaviour in more detail.
Do owls have eyelids?
They do.
Owls have the same basic three-eyelid arrangement, including a nictitating membrane. Cornell’s Great Horned Owl cam has even captured the third eyelid sweeping across the eye while the bird sits on the nest.
For a predator that can strike at moving prey, eye protection is not a small thing.
Do ducks have a third eyelid?
Yes. Waterbirds can use the nictitating membrane while swimming and diving.
It adds protection from water and debris while helping keep the eye surface in working condition.
People sometimes describe it as natural goggles. That is a bit too neat because the membrane does more than one job, but the comparison is easy to understand.
Do woodpeckers use the third eyelid?
Woodpeckers have strong eye protection too.
During rapid hammering, the nictitating membrane can close across the eye. It helps shield the surface from flying wood chips and debris.
That is one small part of a body built around repeatedly hitting wood harder than most backyard birds ever would.
Do birds have eyelashes?
Some do, although they are not eyelashes in the mammal sense.
Human eyelashes are hairs. In birds, eyelash-like structures are modified feathers or bristles around the eyelid.
Cornell researchers have found these structures across a number of bird families. They may help keep particles away from the eyes, especially in species exposed to dust, sand, vegetation, or other debris.
So if a Secretarybird looks as if it has excellent eyelashes, you are not imagining the effect. The material is just different.
Why don’t most birds look like they have eyelids?
Feathers hide a lot of anatomy.
The eyelids are small, and blinking is fast. Unless a bird closes its eyes for a longer rest or you are looking through a camera lens, you may never notice the structures clearly.
The third eyelid is even easier to miss because it can sweep across in a fraction of a second.
Can birds close one eye at a time?
Yes.
That becomes especially interesting during rest. Some birds can keep one eye open while allowing the opposite brain hemisphere to sleep more deeply.
Flock position can even influence which eye stays open in species that use this strategy.
Bird sleep is far stranger than it first appears. We touch on another unusual part of it in can birds sleep while flying?.
Can a bird see through its third eyelid?
In many species, at least partly.
The membrane is often translucent rather than completely opaque. It protects the eye while allowing some light through.
How clear that view is depends on the bird and the membrane.
It is protection first, not a replacement for normal open-eye vision.
Is the third eyelid only found in birds?
No. Nictitating membranes occur in several animal groups, including reptiles and some mammals.
Birds have made especially visible use of it because their eyes are so important for flight, feeding, navigation, and predator detection.
Frequently asked questions
How many eyelids do birds have?
Three on each eye: upper, lower, and the nictitating membrane.
Do birds blink like humans?
They blink, but not always in the same way. Birds also use the sideways-moving third eyelid.
Why does a bird’s eye sometimes look white in a photo?
The camera may have caught the translucent nictitating membrane while it was moving across the eye.
Do all birds have a third eyelid?
Yes. The nictitating membrane is a standard part of bird eye anatomy.
Keep following the question
Read can birds see in the dark?, can birds see colour?, or head back to the Bird Facts hub.