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.
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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.
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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.
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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.
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Phoebe |
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.
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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.
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Grackle |
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 Skullsunlimited.com