An earlier post dealt with the eyelids of the Spotted Dove (Streptopelia chinensis) where it was mentioned that the upper eyelid can only be lowered half way over the eye. The closing of the eye is thus achieved by moving down the upper eyelid to meet the lower eyelid that moves up to the halfway point (below left).
Subsequent images obtained when the Spotted Doves were indulging in allopreening show that the upper eyelid can and does move lower than the halfway mark, contrary to what the literature says. In this case the lower eyelid does not come into play in the closing of the eye (above right). A point to note is that the upper eyelid is translucent. Thus when the eye is closed as a result of the dropping of the upper eyelid, the bird is still able to see what is going on around.
The image on the left shows that the lower eyelid has moved upwards to completely cover the eye. Note that the fringed edge of the upper eyelid is visible, nearly touching that of the lower eyelid. It is interesting to note that the lower eyelid is made up of two parts – an upper translucent half and a lower opaque half. There is a prominent border between the two halves. As with the upper eyelid, the lower also allows the bird to see when the eyelid is drawn up the eye. The image above-left similarly shows the lower eyelid making up of two parts, except that the lower portion is narrower because it has not been raised to its full potential.
I am not an ornithologist or an avian morphologist. And I have not been successful in seeking out any literature on the eyelids of the Spotted Dove. My interpretation of the morphology of the eyelids is based solely on my images. It would be ideal to be able to examine a specimen, either live or dead, but the opportunity did not arise. As such, I welcome alternative interpretations.
I have also not been able to detect any nictitating membrane in any of my images. Does the nictitating membrane exists in the Spotted Dove? Can anyone provide any information on this?
Check out this post on the eyelids of the Brown-throated Sunbird.