Dr Jonathan WK Cheah managed to capture an image of the Plaintive Cuckoo (Cacomantis merulinus) showing the nictitating membrane moving half-way across the eye just before the bird took off from the ground (above right). Obviously the bird spotted a prey and was about to pounce on it. The images on the left shows the normal eye.
Birds have three eyelids – one upper, one lower and a third, the nictitating membrane. This membrane is between the two other eyelids and the cornea and moves sideways. It is used to clean and protect the eye.
When flying, the bird cannot afford to close its eyelids as this leads to loss of vision. The nictitating membrane protects the eye when the bird lunges into the undergrowth in search of prey. It also functions when the bird is under water going after fish.
An earlier post shows the nictitating membrane of the Buffy Fish Owl (Ketupa ketupu).
Images by Dr Jonathan WK Cheah.