Anatomy 4 - Ideal Sight, Part 3

Birds may be the most visually adept, and the most visually dependent, of all creatures on earth.

Black-capped Chickadee

It's astonishing that their eyesight can adjust quickly enough to navigate on the wing through twiggy airspace, focusing both near and far on a desirable flight path.

They can instantly survey distant terrain and distinguish minute morsels at close range. Clearly their eyes have some adaptations different from ours.

Black-throated Green Warbler

Part of their acuity results from relatively large eyes for their size and the proportion of color-sensitive cone receptors on the retina. This isn't just a matter of sharpness. Their spectrum of perception includes part of the ultraviolet range. Pigmented oils within the cones act as color filters. These factors combine to help a warbler distinguish prey otherwise well-blended on its background.

Brown Thrasher

At the center of a bird's eye is the black pupil, a "hole" where the light passes through. Its iris, also usually black or brown but here yellow, is a set of muscles that regulate the diameter of the pupil and the amount of light reaching the retina. The pupils of birds open and close in a rapid and complex manner. In mammals this is an involuntary response to available light, but birds may have the ability to dilate voluntarily.

Red-breasted Nuthatch

Their unusually rapid accommodation to changing focus between distant and close objects is achieved by special constructions in the anatomy of their eyes. Their iris musculature is finely developed above a soft lens. A ring of small overlapping bones stabilizes the eyeball while the lens is being pushed and squeezed. Birds are 'athletically' endowed to versatile vision.

Black Guillemot

These endowments are highly variable based on specializations in their lifestyle. Deep-diving seabirds need to be able to find prey in dim light conditions.

Red-throated Loon, juvenile

Loons, like owls, favor light-and-motion-sensitive rods on their retinas and a relatively small proportion of color-sensitive cones.

Least Flycatcher

The ability of flycatchers and swallows to zero in on tiny distant prey is almost beyond belief. Their bulging eyes make them readily distinguishable from other songbirds.

Peregrine Falcon

At the apex of acuity are the raptors that locate prey from afar, often from great heights. They all have outsized eyes with a relatively flat lens and long focal length that projects a large image onto the retina with a very high concentration of cones. 

Specialized visual adaptations is a hallmark of avian eyesight. The ideal is as adaptable as the lifestyle it fits.

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 Thanks particularly to Chris Leahy for an informative essay in The Birdwatcher's Companion, 2004.