A pair of Peaceful Doves (Geopelia striata) landed on a branch just after a light shower the other evening. They sat there, sometimes apart, at other times close together. They were obviously indulging in a little rest and some comfort behaviour, preening.
What caught my attention were their eyes. The image above-left shows left bird has its eye closed while the right bird has its eye open. The image above-right shows the left bird now has its eye open but covered with the nictitating membrane, the third eyelid. The eye of the bird on the right is now closed.
When the eye is opened, the bird is obviously alert to its surroundings. When the eye is covered with the nictitating membrane, the bird is still alert, as it can see through the membrane. A closed eye does not mean that the bird is oblivious to its surroundings. Notice that the eye is not totally covered by the eyelids. There is a narrow slit that allows the bird to see. So the birds are resting but fully alert.
I was half expecting the pair of doves to remain on the branch for the night. But I was wrong. Within about half an hour, it was then 6.30 pm, they flew off, probably in search of a less exposed perch for the night.