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.

·  ·  ·

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

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.

Anatomy 2 - Ideal Sight, Part One

 

In the last Note we admired the Green Heron's remarkable neck. Its eyes direct that hunting prowess. They're positioned forward and are angled down past the tip of its beak toward prospects for a meal.

Green Heron and Black Duck

That eye configuration is not universally advantageous, however. In the very same environment ducks depend on an entirely different ocular arrangement.

The duck's filter-feeding strategy does not rely on binocular vision to guide its bill. Dabbling goes more by feel than by sight. Its eyes are conveniently located high above foraging and peripherally where they can stay alert for trouble.

Double-crested Cormorant and Green Heron

The eyes of a cormorant, like a heron's, are positioned to pursue a fish right into its bill.

Red-tailed Hawk

Soaring raptors also optimize binocular vision for locating prey from high in the air, and for judging distances.

Merlin

On the other hand this small falcon's eyes are located more laterally. While hunting from a perch or on winged patrols low over the ground, its relatively side-looking cone of vision serves it well.

Barred Owl

Owls, with a human-proportioned face, have maximized the advantages of forward eye positioning for their low-light hunting strategy. This highly specialized vision helps them pinpoint prey that they initially locate by sound. They forego some sight capabilities that other birds depend on. 

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

Anatomy 1 - The Neck Is a Lie

Here comes an adventure story that begins with curiosity about what I assumed was the telescoping neck of a Halibut Point favorite, the Green Heron.

Other than its long legs and beak it seems to have normal proportions for a bird.

Suddenly, from that smooth cloak of feathers, comes a spectacular thrusting neck.

Green Heron at rest

How does it accomplish that? I consulted my brother the neurosurgeon, who doubted the possibility of a telescoping function. He reasoned that extensor movement in all of us vertebrate creatures works on a pulleys-and-ropes principle. Despite outside appearances, something of that sort was going on.

Anatomical drawing of an Anhinga

I went online to see what I could find out about Green Heron necks. The quest led to this illustration, by golly, of pulleys and ropes operating the neck of an Anhinga, another fish-seeker.

In flight a Green Heron does show a serpentine silhouette resembling other long-necked birds. When perched or hunting, however, this small bird minimizes its visibility to both predators and prey.

I had found the anatomical drawing in a blog titled The Neck Is a Lie, a delicious account by Emily Willoughby, self-described "paleoartist and wildlife illustrator." She says, "This blog is intended as a place to put my art, and to sometimes wax scientific on a variety of topics that interest me, most of which will ‒ you guessed it ‒ relate to birds and/or dinosaurs." Irresistible, right? 

You can go further down the trail to the source of her argument that the skeletal structure of animals is often at odds with its physical appearance, on a website by dinosaur aficionado Matt Wedel titled SVPOW.com  (Sauropod Vertebra Picture of the Week). More irresistible stuff.

Our fish-spotting Green Heron comes out if its crouch when it senses a prospect coming near.

It employs that long agile neck beneath the water surface, keeping itself mostly dry.

If that neck is a lie, it's because beauty can be lethal on the interface of life and death.