Egret’s Blink

Snowy Egret (Egretta thula)

My recent focus on a Great Blue Heron’s blink got me curious about the blink of this Snowy Egret. The bird was more than cooperative. It stood at the water’s edge on the east side of the park, held quite still for minutes on end, and paid no attention to my tripod setup. In the video above, the first three blinks are at normal or “real time” video speed, uploaded at 30 frames per second. Each blink takes about 5 frames, or one sixth of a second. The video then goes on to show successive blinks in slow motion. With this technology it’s easy to see that the bird’s transparent nictitating membrane wipes the pupil from front to rear. This is exactly the same process as in the heron. However, the egret’s membrane is colorless, or possibly slightly amber, where the heron’s was tinted light blue. The egret’s blink also appears faster than the great blue’s, perhaps twice as fast. On most of the blinks, the egret’s eye also displaces just a bit and sometimes the pupil changes dilation, so that the blink appears as a coordinated motion of membrane and eyeball. However, the eye movement is very slight. Most birds, including this egret, cannot pivot their eyes and must move their heads to change their field of vision. This video records successive blinks, deleting the time between. Not all of the blinks in the video are identical. One appears to be almost double. Another blink wipes only half the eye.

The bird’s vision was directed at the margin between water and stones. At low tide, this egret and many others hunt by pacing in the mud. At high tide, as here, the bird seemed to be looking for little fish or other creatures that feed on the marine vegetation growing on the rocks.

Snowy Egret (Egretta thula)

About bird vision: Wikipedia
About Snowy Egrets: Cornell Audubon Wikipedia In Chavez Park

Similar Posts:

2 thoughts on “Egret’s Blink

  • Pingback: Willet’s Blink

  • I slowed down the playback of your slo-mo to 1/4-speed, and watched the close-up of the eye (only; head immobile) move with each blink.

    About “… the egret’s eye also displaces just a bit …” and ” Most birds, including this egret, cannot pivot their eyes and must move their heads to change their field of vision”, that seems to be a tough call.

    This egret’s eye appears to move a bit more than “just a bit”, and one must ponder whether that amount of movement is sufficient to allow the egret to ‘look around’* just a sufficient bit for detecting/following prey coming into, moving through, and leaving, the field of view, all without any need to move its head (and potentially frighten the prey with a full head movement).

    * (the eye movements were mostly for/rearward, but some were slightly up/downward as well; and those movements did not appear to be strictly correlated with the movement of the membrane –the eye sometimes moved while the membrane was not actually moving.)

Leave a Reply

Your email address will not be published. Required fields are marked *

Translate »