Thursday, June 19, 2014

The Eye of the Hunter

Positioned between brain and beak, a bird's eyes guide it's every movement and meal.  These eyes differ in size, placement, and anatomy according to the lifestyle adaptations of each species.
Magnolia warbler
Migrating warblers pass through Halibut Point just as spring-green caterpillars are hatching among the spring-green leaves. Where we employ three sets of sensors to make up our color world, most birds have four or even five.(1) The high concentration of red- and green-sensitive cones in their retinas enables warblers to find tidbits in the verdure more easily than we can.
Birds also have colored oils in their light receptors that filter out certain wavelengths to sharpen others. The proportions vary from species to species to give particular advantages.  Just as the yellow tint of aviator sunglasses sharpens clouds against a blue sky, a warbler benefits from red filters to enhance differentiation among greens.

Brown thrasher
A thrasher's eyes are positioned on the side of its head as a compromise between vigilance and foraging. This location, and the somewhat flattened shape of its eyeballs, give it a panoramic but monocular view in all directions for maximum safety. Its eyes can focus together only where these paired fields of vision overlap directly to the front. In this overlap zone the bird has its best judgment for flight maneuvers, and for the use of its  beak, the critical meal-catching spot requiring good depth perception made possible by the binocular vision that we humans  emphasize  in our reliance on hand-eye coordination.

Snowy egret
As befits a hunter of fast-moving prey, an egret's eyes are oriented frontally for increased concentration on minnows. The penalty it pays of diminished sight to the rear has been an acceptable tradeoff in egret  habitat where predators would have to cross wide-open spaces undetected.  

Light bending at the water surface confuses our eyes on the precise location of objects underneath. Egrets are able to correct for refraction. Their strikes are most successful when made at an acute angle, where they seem to be less detectable to fish.  

Seeing and snatching bugs in midair is the sustaining life of flycatchers like the phoebe. Notice the protruding eye lenses for superior scanning and binocular overlap. The lateral placement of its eyes, and specialized receptor areas, enable it to track images to either side independently. 

Birds that hunt on the wing tend to have retinas highly enriched with cone sensors, several times that of humans. They can process images more quickly than we do into a series of discreet sights that to us may appear as an undifferentiated picture, like seeing individual film frames in slow motion.

Pigeon skull (2)
Birds' eyes take up a larger portion of their heads than any other creature. Surrounding tissues reveal only a small part of the bird's eye to the outside world. 

To save space for optical developments birds have given up the muscles that allow us to move our eyes from side to side or up and down. Neck muscles move their whole head instead. As food gulpers rather than chewers they streamline their skulls without teeth and large jaw muscles.

Grackles search the meadow with one eye to the ground and the other cocked to the sky. Their brains synthesize bilateral information in their own version of multi-tasking, then converge the overlap binocularly to peck or fly. 

This hunter's posture is all purpose and focus, leaning into the prospect of taking a life to carry on its own. The full force of its intent lies poised behind the eye in a chilling acknowledgement of survival equations. The eye reveals the bird's nature as acutely as it guides its actions.

(1) Physiology resources: Wikipedia article "Bird Vision"; and the website "How Birds See their World" by Dr. Esteban Fernandez-Juricic, Purdue University  

(2) Image from

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