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Why Birds Have Nictitating Membranes

Find out the value of these ghostly third eyelids for birds of prey.

Brian L. Sullivan

A great look at a Bald Eagle's nictitating membrane in action in this video from The Raptor Resource Project. 

I eased my car down the snow-dusted road, intently scanning for eagles on a blustery winter day at Upper Klamath National Wildlife Refuge in Oregon. There to study raptors, I hoped to photograph the various ages of Bald Eagles that gather at the refuge to feed on a bounty of wintering ducks and waterfowl. 

Ahead I saw a massive shape perched on the roadside fence — my chance to photograph a perched adult Bald Eagle. As I pulled the car alongside the bird, it barely noted my presence, just eyeing me casually before returning to its scrutiny of a nearby flock of Tundra Swans

I stayed in the car and used it as my photography blind, shooting from the open window. The eagle hopped onto a sprinkler head to get a better view of the swans and simultaneously gave me a better opportunity for photography. 

After five minutes with the bird, I happily drove off in pursuit of several nearby Rough-legged Hawks. Upon returning home and reviewing my photos, however, I felt shocked and dismayed to find that most of the eagle shots were marred by the dreaded third eyelid. 

Nothing quite ruins a shot like a bird's ghostly third eyelid, such as in this shot of a Golden Eagle. 

The Nictitating Membrane
This gossamer shield, known as the nictitating membrane, helps birds clear their eyes of dirt, dust and debris. For many wildlife photographers, the nictitating membrane can turn a majestic bird into a possessed-looking avian sub-demon. There’s nothing worse than nailing the shot of a hawk in flight with everything perfectly in focus, only to find later that the milky white membranes covered the bird’s striking eyes.

Most birds have nictitating membranes, but it appears especially obvious in birds of prey. Why might this be? Raptors rely heavily on eyesight; they need their famously powerful vision to hunt prey, so they have to keep their eyes in good condition. 

Like many raptors, Rough-legged Hawks sit on high perches and scan the surrounding terrain for prey, often during inclement weather such as high winds, rain and snow. These conditions expose their eyes to the worst of the elements, and they use the nictitating membranes to regularly sweep debris from their eyes. 

An accipiter crashing through dense tangles in pursuit of a landbird could easily scratch its eyes on a branch. A Red-tailed Hawk grappling with a snake requires safety goggles to offer some measure of protection. A Peregrine Falcon in high-speed pursuit of its prey can get "dry eye” due to air passing over its eyes at nearly 200 mph. The nictitating membranes handle all of these scenarios. 

Unlike human eyelids, nictitating membranes can close to protect the eyes yet still allow reasonably good vision. While our eyelids move up and down, the nictitating membranes move horizontally across a bird’s eyes, sweeping from the front to the back. 

The nictitating membrane of a bird's eye helps protect their eyes, and it is valuable to birds of prey, such as this Rough-legged Hawk.

Birds generally don’t blink like humans do. Instead, they blink with the nictitating membranes, which moistens their eyes while clearing debris. Birds use their other eyelids when sleeping — and mostly close their eyes from the bottom eyelid upward.

In addition to birds, many reptiles and some fish, notably sharks, have prominent nictitating membranes. Perhaps you’ve seen the slow-motion images of Great White Sharks attacking their prey with cold black eyes that appear to roll back into their heads and expose the whites below. In reality, you’re seeing the white nictitating membranes being activated to protect the sharks’ eyes during the attack. Most mammals have just vestigial nictitating membranes, visible only if you pull back the other two eyelids. 

Next time you’re photographing birds and have the chance to observe one up close, take lots of photos. You’ll probably capture the bird mid-blink, and you can see for yourself how the nictitating membranes help birds keep their eyes in top shape.

Want to learn more about raptors? 

Why Gyrfalcons Rule The Arctic
Raptors Conservation

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Posted: July 22, 2014, 2:15 p.m. PDT

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