Anatomy 3 - Ideal Sight, Part 2

 

Cedar Waxwing

Have you noticed that when birds look at you‒and that's generally before you see them‒they're in profile?

Blue Jay

With a fellow human, looking away diminishes contact and seems unreceptive.

Redpoll

Birds, on the other hand, are giving you their best attention by turning their heads sideways. They're finding out what they need to know about you with just one eye. Generally they are not doing this with the depth-of-field advantages of binocular vision. Their emphasis on peripheral vision renders adequate detail in the center of its cone nevertheless. 

Grackle

What is the other eye doing? This grackle can simultaneously look for morsels in the meadow while scanning for movements of friend and foe in the sky. Its brain 'reads' and integrates optical information differently than ours.

Mourning Dove

Birds that walk with a distinctive bobbing motion of the head are trying to keep their eyes at a consistent unblurred focal distance from foraging opportunities on the ground as their bodies go up and down with each step. ¹

Eastern Screech Owl

Birds' eyes are less mobile than ours, their necks more flexible. The extreme adaptation of this sort is the owl that can only stare straight ahead with its binocular vision but is capable of extraordinary cervical rotation. Because it is a predator itself, and minimally exposed to danger in daylight, it has reduced its need for peripheral vision.

Pigeon skull

What you can see of the bird's eye is only a small fraction of its eyeball. Most of it is housed within the skull, devoted to optical acuity. There's no room for the extent of rotational muscles that we have, hence the need for an agile neck. Compared to other animals birds have proportionally very large eyes that frequently outweigh the brain. ² Anatomically these eyes give the impression of an elaborated extension of the brain, capped by a lens.

Great Egret

Combined optical and neurological features allow wading fish-seekers to compensate for light refraction at the water's surface, to accurately plan their strike.

Belted Kingfisher

Diving birds have to have make similar calculations when they plunge.

Purple Sandpiper

Migrating birds are known to rest parts of their brain in flight. This sandpiper is resting with one eye on guard. In order to keep their eyes almost constantly open birds depend on a transparent moistening membrane that traverses from underneath the eyelids. Except in sleep they always want to see their environment. 

Varied anatomies are crucial to success in particular niches and strategies. There is no single visual ideal. 

Sources

1. Dr. Esteban Fernandez-Juricic, Department of Biological Sciences, Purdue University

2. Chris Leahy, The Birdwatcher's Companion, 2004.