Bald Eagle’s Nictitating Membrane

Bald eagles, like other birds of prey have incredible eyesight. To keep their vision sharp, bald eagles clean their eyes with the help of an inner eyelid called nictitating membrane. Besides protection, the membrane keeps the eye moist and clean of dust. It slides horizontally from left to right every three to four seconds. And, by being transparent, the eagle never loses sight of the prey even when the membrane is drawn across the eye.

Compare the bald eagle eye in the photographs below with the eyelid on and off.

This bald eagle is a three year and a half old female named Pandora. The bird belongs to Stephen Bucciarelli from Predator Bird Services. We met Pandora during the Wings of Spring Festival in Rondeau Park. We estimate that during Stephen's inspiring 1-hour long presentation, Pandora blinked her inner eyelid approximately 1000 times.

It takes about five years for the Bald Eagle to reach maturity and to gain its distinctive white head and tail that everybody recognizes. For this reason, young bald eagles are easily confused with golden eagles, especially when observed from a distance. Learn the clues provided by our readers on how to correctly identify the young bald eagle from the golden eagle.

As an interesting fact, birds are not the only species that have a nictitating membrane. Reptiles, sharks, amphibians, and some mammals such as polar bears have it too.

Let's take a closer look at Pandora and see the nictitating membrane at 100% magnification. Note the blood veins traversing the eyelid in great numbers.

We hope to meet Pandora and Stephen again. Till then we wish them both good 'hunting'.

• The nictitating membrane of the Sandhill Crane
  
• The nictitating membrane of the Red-Tailed Hawk