Anatomy 5 - Feet

Most of the muscle mass of a bird is in close to its core for reasons of aerodynamics, self-protection and warmth. The bulk of its leg is normally invisible to us beneath feathers. The foot emerges into sight at the ankle, a backward-bending joint, like ours, that we could easily mistake for its knee because of its location high up on the leg. From there the toes fan out in many versatile configurations that adapt the bird to its lifestyle.

Ruby-throated Hummingbird

Most birds we encounter away from the shore use their feet for perching and walking. Hummingbirds, on the other hand, spend much of their time and energy hovering. Their physique emphasizes wing muscles over legs. Their feet are notably small.

Brown Creeper

The relatively large feet of another tiny bird suit it for clambering up and down tree trunks in search of insects. Brown Creepers depend on their long, strongly curved rear claws to help with stability and maneuverability. Their legs are much more developed than hummingbirds.

Turkey

At the upper size limit of Halibut Point birds are turkeys that browse on the ground. They have to be wary and able to outrun foxes and coyotes. Turkeys are very fast, but those hefty leg muscles built up from foraging and running make them a prize catch for carnivores, hunters, and domesticated poultry producers. Partly in compensation for this vulnerability turkeys have developed  sharp, bony spurs on their heel that they deploy in aerial attacks against rivals and in self defense.

Downy Woodpecker

The upland birds in our realm almost exclusively have four toes, three pointed forward, one to the rear. Woodpeckers have found an evolutionary advantage in developing a two-forward, two backward combination for anchoring to a tree. Chimney swifts have shifted all four claws to the front to facilitate hanging at rest inside their roost.

Semipalmated Plover

The water's edge offers the greatest variety of foot adaptations. Semipalmated means partially webbed which improves this plover's traction on slippery rocks and quite possibly its 'swimming' movements through shallow water.

Red-necked Grebe

Grebes are able to work their lobed toes as independent paddles for both propulsion and steering in the pursuit of fish. They can twist their foot 90˚ on the forward stroke to minimize drag through the water.

Double-breasted Cormorants

Cormorants are one of the few birds with webbing extending to all four toes. Coupled with their enormous feet they can reach and accelerate quickly in submarine depths.

Mallards

Ducks of all sorts, being dabblers or shallow divers, manage well with webbing limited to the three forward-pointing toes. For an explanation on their ability to stand comfortably on ice, see my previous posting Cold Feet.

Snow Bunting

Another approach to avoiding heat loss through the feet is to bundle up, as this cool-weather visitor to Halibut Point does with tufts of feathers all the way to its toes. Snowy Owls have developed this protection even further.

Peregrine Falcon

The aerial hunters known as raptors all have large feet capable of tremendous crushing force and piercing claws.

Great Blue Heron

Herons have large feet with very little clenching power, to support then on marshy conditions.

Their long toes radiate like spokes on a wheel.

Black-crowned Night Heron

Certainly there is not 'one size fits all' with wading birds. The arrangement‒ and uneven length ‒  of toes on the Black-crowned Night Heron, which often hunts from a perch, gives a compromise solution to its niche at the water's edge.

Oystercatcher

Oystercatchers, with only three toes, occasionally visit our shoreline, showing one of their particular adaptations to foraging on rocky beaches.

Noticing birds' diverse feet is another way to marvel at nature's ongoing adaptations to opportunity.

Anatomy Study Break - Squirrel Olympics

In the midst of these recent feature stories on the makeup of birds we can divert for a bit into awe and hilarity with Mark Ruber who produced a YouTube on his contest with squirrels raiding his daughter's bird feeder. 

Squirrels flying through the treetops come as close as any mammal can to the high-speed aerial agility of birds. What they lack in wings they surpass with puzzle-solving skills and fearlessness.

Mark began to learn about the well-endowed adversary he was up against when he bought a series of supposedly squirrel-proof feeders.

His investments were systematically plundered by crafty, athletic squirrels. He got to know and name the four marauders in 'his' yard.

Mark is a former NASA engineer with a garage workshop and a zany sense of humor. He and his buddy decided to make life tough for the squirrels and find out what feats they were really capable of. His wife made them promise not to hurt anybody.

Ever the scientist, he set up a nut buffet to determine their favorite food ‒ and bait. They preferred walnuts in every trial.

At the end of an obstacle course testing their intelligence, stamina and grit he constructed a squirrel's dream.

Each station in the course got progressively harder: The Bridge of Instability, The Maze of a Thousand Corridors, The Pitchfork Tumblers of Treachery, The Homewrecker, The Slinky Bridge of Deception, The Tourist Trap, The Quad Steps of Grid Elevation, and The Orbital Assist Platform. 

Mark videotaped the awesome heroics and shenanigans from a blind, like the big-time sports programs do. He analyzed them in slow motion. 

Pointing to the walnut stash Mark said, "If they make it here they've earned the right to gorge next to the birds for as long as they want." By the end of the week all the squirrels could do the full course in 40 seconds. There were winners in every direction--you, me, him, and the squirrel.

Flying squirrel

Now, back to anatomy class. Mark discovered that catapulted squirrels always land on their feet. He concluded they can survive a fall from any height by spreading their limbs like a parachute, lowering their terminal velocity, then extending their arms and legs as shock-absorbers. 

They instantly know where they intend to land. Mark calculated that within 300 milliseconds the squirrel fixes its eyes on that spot and keeps its head locked on target the whole way down through twists, turns and tumbles.

Its second trick in flight control makes a NASA man marvel. When the squirrel wants to keep his upper body from rotating he puts his arms and legs out. When it tucks them in, its body spins, a technique that figure skaters and ballerinas delight us with. Mark calls it straight up conservation of angular momentum.

At Halibut Point we witness our own squirrel performances in the treetops.

Red squirrel

It might not be as much fun as matching wits with them experimentally at the bird feeder, but these critters generally spark up a walk in the Park.

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.