Nictitating membrane
The nictitating membrane is a transparent or translucent third eyelid present in some animals, including birds, that can be drawn across the eye from the medial canthus to protect and moisten it while maintaining vision. The "medial canthus" is the inner corner of the eye (towards the noise or bill).
Primary use
This membrane helps protect the eye against impacts, e.g. of insects, or while feeding, drinking or fighting. Below an example:
Lateral views of a male
Sacred Kingfisher;
inner eyelid closed, left, eyelid open, right
(photos courtesy of M. Eaton)
The way the membrane normally moves is illustrated very well in this composite by Toby Hudson (source: Wikipedia).
3 views of the left eye of a
Masked Lapwing;
from left to right: Nictitating membrane open, half-closed, closed
(photo © Toby Hudson, Wikipedia)
Other uses
By taking close-up photos, we have discovered that birds can also use their nictitating membranes to shield their eyes against blinding light by only partially deploying them. It is, for example, annoying when a photographer is working with a flash...!
3 views of the left eye of a
Australian Boobook
shielding its eyes by partially deploying the nictitating membranes
Close-up frontal view of a female Magpie-lark in its
nest
[Eulah Creek, NSW, December 2025]
Close-up frontal view of a female Magpie-lark in its
nest;
when blinded by the camera's flash, it closed its
nictitating membranes
[Eulah Creek, NSW, December 2025]
But the nictitating membranes can also be deployed against blinding sunlight:
2 photos of a
Willie Wagtail
looking left to right, with bright sunlight blinding the bird,
taken only seconds apart
Curious detail
Careful study of the bird's eyes in the two photos shown above
reveals that in the photo on the right, as one would expect, the
nictitating membrane is in the upper right quadrant of the eye,
i.e. moving in from the medial canthus. In this position it shields
the eye against blinding sunlight coming in from the right.
However, in the photo on the left the membrane again appears to
be in the upper right quadrant of the eye, shielding it against
bright sunlight coming from that direction - but in this case it
is on the side of the lateral canthum, i.e. in the outer corner of
the eye! At least, this is where the eye looks more milky than in
the other half. Ideas, anyone?
Close-up look at the nictitating membranes of the same Willie Wagtail photos as shown above
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Last updated:
Tue, 17 February 2026, 9:55 +00:00