THE NICTITATING MEMBRANE
In common with all other birds, the dipper has what is effectively a windscreen – a near-transparent third eyelid known as the nictitating membrane, which it draws across its eye. This membrane even acts like windscreen wipers, as it has the extra function of moistening and cleaning the front of the eye. Rapid blinks of this third eyelid while a bird is moving, especially when it is leaning over to feed or forage, suggest that it has an important role in protecting the eyes.