Monday, 16 January 2017

Avian Nictitating Membrane

Birds have the means to clean and protect their eyes through the use of a nictitating membrane. This thin layer of skin slides in the horizontal plane, where the main eyelid closes in a vertical plane. The species that dive for food use this membrane under the surface, protecting the eye. Raptors that hunt prey at speed, such as the peregrine, may close the membrane at point of impact, or during the final stoop, again for protection. Photographs of these birds frequently show the membrane, giving them the appearance of being blinded. It is through that the bird can still see well during the time the membrane is closed.

Even in sheltered bays on LNNR, Northerly gales produce big waves, salt-spray, and sleet. These resting wigeon use their nictitating eye membrane to give them some relief. They also use the membrane to see under the surface of the water. (c) JJD

Hunting for aquatic larvae entirely submerged underwater, the dipper can see clearly through the membrane, which is a striking white colour (Not taken on the Reserve). (c) JJD

The picture (not taken on the Reserve) shows the nictitating membrane partially retracted. The bird excavates and drums with such force that it needs the protection of the membrane to shield it from flying splinters. (c) JJD

(All Text and Photos John Dunn)

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