“I first learned about birds’ ability to sleep with only one eye closed when this was posted: LINK. From Wikipedia, unihemispheric slow-wave sleep (USWS) is the ability to sleep with one half of the brain while the other half remains alert. More information on this phenomenon can be found HERE.
“A Greater Racket-tailed Drongo (Dicrurus paradiseus), a common resident here, was encountered recently. This drongo had lost one of its rackets (above left) and seemed to be following a troop of Long-tailed Macaques. It was just past noon when it decided to perch on a low level branch above a forest trail after the monkeys had left.
“While snapping shots of this drongo, I noticed that something was not quite right with its left eye that was facing me (above). Instead of red, I saw pink. Vaguely recalled seeing it with both eyes normal and red when it was facing me earlier. It was only much later, when it faced me again, that I realised that the pink-coloured eye was in fact its eyelid that was closed (below). It was sleeping with one eye closed and the other eye opened; the phenomenon of unihemispheric slow-wave sleep.
“Some photos showing the drongo with its left eyelid closed. Four of these photos were selected to illustrate that its eyelid had remained closed and not merely blinking (see also the two images below). These photos showed the drongo with its head in different positions but with left eyelid closed.
“Calculating from the timing between the first and last photos with eyelid closed, the duration of the unihemispheric slow-wave sleep was at least 1 minute and 10 seconds. The drongo flew off immediately after it turned its head to its right. Unable to view whether its left eyelid had opened when it flew away.
Kwong Wai Chong
28th JUne 2015