Eye movements of the Brown-throated Sunbird

posted in: Morphology-Develop., Sunbirds | 0

“Attached are cropped images showing the head of a male Brown-throated Sunbird (Anthreptes malacensis) that were taken at close proximity. The nictitating membrane and lower eyelid were captured in action.

“The nictitating membrane, also known as the third eyelid, is believed to be used to protect and moisten the eye. It is either transparent or translucent; thus helping to keep some visibility when it is deployed.

“In the first set of images, the sunbird’s translucent nictitating membrane was visible. It could be seen to commence from the front part of the eye, moving across horizontally past the pupil (above left), before completely covering the entire eye (above centre). After less than a second, it retracted to re-expose the eye to its normal condition (above right).

“The second set of images managed to capture the sequence of the eyelid moving in tandem with the nictitating membrane. [Note the distinct lower eyelid (left) that is not obvious in the image above-left when the eyelid is not about to be raised.] Interestingly, the nictitating membrane was closed before the lower eyelid moved upwards to show a half-closed eye (below left and centre). Less than a second later, the nictitating membrane was retracted to uncover the eye that was not covered by the lower eyelid (below right). However, the white and opaque lower eyelid remained drawn for a relatively longer period of about 3 seconds before resuming its normal open-eye position. The upper eyelid was not seen in action.

“Could this sunbird be catching a wink? The sunbird was somewhat inactive during the brief moment that its lower eyelid was shut. But it resumed to its active chirping self immediately after that. Unfortunately, the other eye was not in view. Hence, whether it was for one eye or both eyes cannot be determined.”

Kwong Wai Chong
Singapore
14th June 2010

Note: The whitish patch of skin appearing as the lower eyelid moves upwards is rather unusual. This patch has a distinct lower border. We sent the images to Dr David R Wells who replied: “Yes; the bird is extending its nictitating membrane (‘third eyelid’) then closing its eye by raising the lower lid (upper lid smaller, less mobile). The pale patch you see below the eye is simply the featherless skin of the stretched lower lid.”

Check out the eye movements in the Buffy Fish Owl (Ketupa ketupu) HERE.

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