Raptor Adaptations
How do they do those things they do?
Adaptations! That’s how. When examining just one species of plant or animal, its shape, behavior, tools, all the things that differentiate it from the rest could keep one busy accounting for their uses for a very long time. To survive in a particular section of an environment, or niche, animals, including ourselves, have developed varying forms and actions that make it possible for each of us to exploit a food source or living space. These variations are our adaptations and they are innumerable. Whether it is the phosphorescent chemical reaction occurring on spots on the skin of a fish living in the deep, dark ocean or the flight response of a deer or the Velcro-like structure of the microscopic parts of each feather on a bird, these adaptations will never cease to astound us. When we watch nature documentaries we are left spellbound at the unimaginable ways in which animals have found to survive. Raptors are no different and have their own amazing tales of form and function; a small but important collection of them is found here.
But first, from The Outermost House by Henry Beston
We need another and a wiser and perhaps a more mystical concept of animals. Remote from universal nature, and living by complicated artifice, man in civilization surveys the creatures through the glass of his knowledge and sees thereby a feather magnified and the whole image in distortion. We patronize them for their incompleteness, for their tragic fate of haven taken form so far below ourselves. And therein we err and greatly err. For the animal shall not be measured by man. In a world older and more complete than ours they move finished and complete, gifted with extensions of the senses we have lost or never attained, living by voices we shall never hear. They are not brethren, they are not underlings; they are other nations, caught with ourselves in the net of life and time, fellow prisoners of the splendor and travail of the earth.
Foot Notes
by
Stephanie Streeter
from DVRC Journal Fall/Winter 1998
A foot is a foot is just a foot. Let’s face it, we don’t give much thought to feet, unless they’re our own and happen to be aching. We especially don’t pay attention to avian feet when watching birds; flashy plumage and feathered wings that whisk a bird skyward in a heartbeat are what catch our eye. But perhaps it’s time to take a closer look at bird feet, raptor feet in particular, because in those often overlooked appendages lie the key to survival for most birds of prey.
One of the ways scientists have classified bird families is by toe arrangement. There are ten classifications, one of which is Raptorial and it includes all condors, eagles, falcons, hawks, kites, ospreys and vultures. The definition of raptorial toes are toes that are heavily padded on the bottom with large, strong, curved claws (talons).
The above description is fine as far as it goes, but it doesn’t begin to touch on the many specialized characteristics that help define each species' lifestyle and type of prey it is most likely to take. Consider the great horned owl. This is a species that is large and powerful, able to take an amazing range of prey. The great horned owl’s diet can include snakes, fish, birds (including other birds of prey as large as the red-tailed hawk,) rodents, muskrats, rabbits, skunks, squirrels, gophers, weasels, opossums, porcupines and bats. According to Arthur Cleveland Bent in The Life History Of American Birds Of Prey, "almost any living creature that walks, crawls, flies, or swims, except the large mammals, is the great horned owl’s legitimate prey." Much of this bird’s power resides in its large, sturdy feet. I can attest to this. All too often, a great horned owl has gotten a foot free and gripped my welders-glove clad hand hard enough to leave bruises from the pressure of its toes alone. Accompanying the bruises were the inevitable puncture wounds from the one inch needle-sharp talons. Welders gloves are poor protection against a determined great horned owl.
Among the raptors, one interesting feature shared by all owls and the osprey is the opposable toe. Most birds, including condors, hawks, falcons, eagles, kites and vultures have their toes arranged in a three facing-forward, one (the hallux) facing-rear arrangement. While owls can and do position their toes in this pattern, they are most often seen with the outermost, smallest toe (digit IV) positioned with the rear hallux (digit I) so that the toes are in a two-forward, two-backward configuration. It is believed that this ability to rotate the small toe gives the owls greater dexterity enabling them to take a wider variety of prey.
I would be remiss not to talk about legs while covering the subject of feet because the strength of a raptor’s foot is provided by well-developed muscles that run along the tibiotarsus, a characteristic seen only in birds of prey. A word about avian legs is also in order to clear up a common misconception. Most people assume the long, scale-covered limb (or feather-covered in the case of most owls, the golden eagle and the rough-legged hawk) directly above the bird’s toes is its lower leg - what on us is the tibia between ankle and knee. Yet if that were so, then the bird’s knees could not bend "backwards" as is believed by many. In fact, bird knees bend the same way ours do. What we are looking at is the foot of the bird, known as the tarso-metatarsus or tarsus. All birds walk on their toes and the joint at the upper end of the tarso-metatarsus is actually the heel of the bird’s foot. What looks to be the bird’s knee is actually its ankle, and it bends in the same manner as ours.
As stated previously, the legs and tarsi of most owls and a few diurnal raptors are feather covered. In some species, like the great horned owl and the snowy owl, the toes are heavily feathered as well. It is thought that this feathering helps to keep one of the most important parts of a raptor’s anatomy, its prey catching legs and feet, warm. Certainly for an Arctic dwelling species like the snowy owl this makes sense. But it also makes sense for other species of owls. Most owls are nocturnal, which means they are active during the coldest part of each day.
One species of owl that does conduct its business during daylight hours is the burrowing owl, and that bird’s unusually long tarsi and toes bear only a light covering of feathers. Owls, unlike most diurnal birds of prey, do not make a noticeable autumn migration southward; instead they remain in or near their home territory where many are subjected to cold winter temperatures. While it is true that many southern dwelling owls have feathered tarsi, it is interesting to note that the few species of owls without feathers on this part of their anatomy are birds of warmer climes, and they carry names that, while inaccurate, reflect this characteristic. There is the Bare-Legged Owl found only in Cuba, and the Bare-Shanked Screech Owl that lives in Costa Rica, Panama and northwestern Colombia. For both of these species, Bare-Footed would be a more precise, although less appealing, name.
The owl family can lay claim to another unusual foot adaptation. The barn owl is the only owl, along with members of the heron, bittern and nightjar families, that has a pectinate middle claw. This claw, or talon in the case of the barn owl, has scales along its inner edge that resemble the teeth of a comb and is used to preen and straighten head and neck feathers. It is also used on the feathers and bristles around the bird’s beak to keep them clean and parasite free. Since this article is about bird feet, it seems the perfect time to answer the often asked question, "Why doesn’t a bird fall off its perch when sleeping?" It doesn’t fall off because of the arrangement of the tendons in its toes. The toes actually tighten around the perch as the bird drifts into sleep. As the bird lowers its body into a resting position, the large flexor tendon or Achilles tendon automatically curls the toes inward tightening the grip which allows the bird to sleep or rest without falling of its perch. This same tendon also allows a raptor’s foot to tighten and hold its prey securely. Once caught, few animals can escape the relentless grip of a bird of prey.
But, what tells a raptor’s foot it has made contact with prey and that it should immediately close its grasp? Some ornithologists believe the answer lies in the Herbst’s corpuscles which are found in a bird’s toe pads. These corpuscles are very sensitive to vibration and therefore increase the bird’s sense of touch. The instant a raptor’s foot touches prey, the Herbst’s corpuscles cause it to clamp shut. This theory is strengthened by the fact that large amounts of Herbst’s corpuscles are found in the bills and mouths of birds that search for food with their bills like the skimmer and the sandpiper.
Many birds of prey can be identified by their feet alone because of specialized characteristics that have evolved to help them catch their preferred prey as economically as possible. The osprey is one such bird. This is a fish eater, a bird that dives feet-first into rivers and lakes, sometimes completely submerging its body to secure its meal. Its opposable toe allows it to more firmly grip its slippery, wiggling prey when flying to shore, and the fish is always carried in the same manner - facing head first to provide a streamlined shape for easier flight.
Each of an osprey’s toes is armed with long, sharply curved talons that resemble fish hooks, another adaptation peculiar to this unique raptor. Even the bottom of an osprey’s toes are designed for fish catching. They are very rough to the touch because of the small wart-like projections called spicules that cover them. This sandpaper-like texture gives the bird a surer grasp.
Another distinctive foot type belongs to the accipiter family, the sharp-shinned hawk, Cooper's hawk and goshawk. These are forest dwelling birds that put other birds at the top of their menu - primarily songbirds and upland game birds. The hawk raiding your bird feeder is most likely either a sharp-shinned hawk or a Cooper's hawk. The toes of each of these birds are long and slender in comparison to the toes of other raptors of similar body size. An accipiter’s toes and its equally long and slender tarsus are designed to catch birds. The long toes can easily slip beneath plumage to securely grasp avian prey, while the long legs allow the hawk to thrust its foot into thick brush where fleeing passerines often seek shelter. Those long legs also make it possible for an accipiter to slip its foot down a burrow hole to catch a chipmunk or squirrel it is pursuing, a feat I’ve watched deftly performed by a goshawk.
The peregrine falcon is another long toed raptor. Its toes are slim like an accipiter’s and have very pronounced toe pads. Unlike the accipiters, it has noticeably short tarsi. This combination of long toes and short tarsus distinguish the peregrine’s foot from other raptors'. Tom Cade’s book, Falcons Of The World has a table on foot proportions which lists the middle toe length as a percentage of tarsal length x 100 for various falcons. The North American peregrine’s statistics are male - 93.9%, female - 99.1%; the European peregrine’s, male - 100.6%, female - 100.4%.
Unlike other raptors, the peregrine does not use its feet to kill its prey, instead, it uses its long toes to securely hold down its quarry so it can dispatch it with a quick bite to the neck. The high flying peregrine does, however, use its feet to literally knock its prey out of the sky. Its preferred food is other birds, often much larger than itself, and it hunts them in a most spectacular manner. The peregrine first positions itself hundreds of feet above the bird it is hunting, then with folded wings, hurtles itself head-first towards its prey at speeds that can reach 200 m.p.h. Just before it makes contact, the falcon rights itself and strikes the bird, knocking it to the ground. But, just how the peregrine strikes its quarry is a matter of some disagreement. Dr. Tom Cade, a master falconer and the scientist responsible for creating the Peregrine Fund which reintroduced the peregrine to areas in the U.S. where it had been wiped-out by the pesticide DDT, claims the peregrine hits its prey with an opened foot, often raking the rear talons across the bird’s back, forcing it to the ground. He feels this is advantageous to the peregrine because with toes extended, the falcon can either strike or grab its prey as the situation warrants. Many falconers, who have had years of experience flying peregrines to game, swear that the peregrine always hits its prey with closed feet. Their reasoning, and that of other scientists, is that a closed foot, like a closed fist, will knock any bird from the sky when delivered by a falcon traveling 200 m.p.h.; additionally, a closed foot will prevent the peregrine’s long toes from being broken during impact. No matter who is right, the peregrine’s hunting style remains one of the most breath-taking in the animal kingdom.
At the other end of the spectrum are the buteos. These hawks have solid, robust, chunky bodies, with feet to match. The red-tail hawk, the most common of our northeastern buteos, is capable of taking a wide variety of prey, but like the other buteos, eats mainly rodents. Once this large raptor has grabbed a mouse or vole, the end is a foregone conclusion. The red-tail kills its prey, whether rabbit or mouse, in the same manner. It sinks its talons into the animal’s body as soon as it has been caught. In the case of a mouse one foot will do, for a larger animal both feet are used, with one foot securing the head, the other the body. While this sounds cruel and unusually painful, especially if the prey does not succumb at once, it is not unduly so. When the hawk sinks its talons into the prey, the animal almost immediately goes into shock. The animal dies more quickly of shock rather than slowly as a result of its wounds. Death in the wild is not a gentle peaceful thing, but it is, at least, relatively quick when induced by a raptor.
Of all the birds of prey, the vulture’s toes least meet the criteria for raptorial classification. They are weak, poorly padded and have only slightly curved talons. A vulture’s foot looks more like a turkey’s than a hawk’s. But again, the foot reflects the life-style of the bird. This is a scavenger and as such does not have to catch and kill its prey. What it must do is securely balance itself on a carcass so it can feed. The vulture’s large feet get the job done, but in the process often get dirty. Cleaning them, however, isn’t as difficult as it would be for a great horned owl, for example, because the vulture’s toes and tarsi are completely featherless.
A habit, unique to turkey vultures is that of defecating down their legs and toes to cool off. It is also thought this odd behavior helps to kill bacteria the bird may have picked-up while walking on, and in, their less then fresh choice of food.
By rights, I should not be talking about vultures, at least New World vultures like the turkey and black, when referring to raptor feet, because in 1994 the American Ornithologists Union reclassified them as part of the stork family. According to the Union’s chairman of the nomenclature committee, Burt L, Monroe, Jr., "Vultures are nothing but short-legged storks." That may be so, and perhaps one of the anatomical features that helped sway the scientists’ minds were the New World vulture’s unraptor like feet. However, Old World vultures are still classified as raptors and they too have feet that look more like a gallinaceous bird’s than a bird of prey’s. But, for the vultures, both Old World and New, they work.
Short toes, long toes, sturdy toes, slender toes, toes with spicules, toes with feathers and toes without, talons with comb-like scales and talons shaped like fish hooks - with such variety, raptor feet are certainly worth a second look, and just maybe, a third.
from DVRC Journal Spring/Summer 1999
Beaks come in all shapes, from the crossed mandibles of the aptly named crossbill, to the spoon-shaped beak of the equally appropriately named spoonbill. They also come in variety of sizes from the tiny beak of the whippoorwill, which hides a deceptively large gaping mouth, to the long thin sensitive beak of the woodcock. This very diversity is one of the features that tells ornithologists and bird watchers alike to which family a bird belongs. If it looks like a duck, quacks like a duck, and walks like a duck it must be a duck. An old saw, but true, and one of the reasons it looks like a duck is because of its distinctive duck-like beak. So, beaks are a handy identification feature, but, they are much more than that. Each type of bird has a bill that has evolved to suit its lifestyle. The skimmer has a beak with a larger lower mandible that it uses to scoop up food as it skims it across the water while in flight, while the woodpecker uses its strong beak to find food by drilling holes in trees.
The beak is involved with eating and, therefore, survival. For the birds of prey, that means they must have a beak which will allow them to break into and consume prey. The strong, sharply hooked beak which distinguishes all raptors does just that. But, even among raptors, there are differences. Before talking about those differences, however, we should first take a look at the structure of the beak.
The bill of the bird, properly called the rostrum, is a two-part structure made up of an upper and lower beak or mandible. More than a dozen bones form the skeletal structure of the beak. Both mandibles are made up of many bones that have fused together. The cutting edges of the upper and lower mandibles are called the upper mandibular tomia and the lower mandibular tomia. The raised center line that extends from the tip of the bird’s beak to its forehead is known as the culmen.
On most songbirds, the nostrils or nares, are located on either side of the culmen at the base of the upper mandible near the feathered forehead. For some species, like pigeons and doves, the nostrils are situated in a soft-to-the-touch fleshy covering at the base of the upper mandible called the operculum. Operculum is taken from Latin and means lid. The nostrils of raptors are also found in a fleshy protuberance at the base of the upper mandible and it is known as the cere. The use of operculum and cere is sometimes interchanged, although they should not be because technically there is a difference. The cere of the raptors is hard, and most often yellow. The word cere is also taken from Latin and means wax, perhaps because the cere has a waxy look, while the operculum appears more flesh-like, both in color and texture.
Unlike humans, birds do not use their nostrils for smelling. Most birds possess a very poorly developed sense of smell; but, they have little need of it for they spend most of their time in the air where there are few odors. Smells are more intense at ground-level because the gasses of volatile substances that produce them are heavy and quickly sink. It is not surprising to learn then that it is the ground-dwelling species, like ducks, snipes and kiwis, that have relatively large olfactory lobes and consequently, the best sense of smell. One exception to this ground-dwelling rule is the turkey vulture. Turkey vultures are the only vulture that has been proven to locate food by smell (Stager 1964,1967). In Stager’s experiments, turkey vultures located hidden animal baits, but ignored an easy to see, life-like stuffed mule deer.
 Not only do turkey vultures have the unique ability, among raptors, to locate food by smell, they also have unusual nostrils that can be seen through from one side to the other. |
Many waterbirds including ducks and geese have a hard downward pointing protrusion on the end of their upper mandible called the unguis, or more commonly, the nail. At the time of hatching all birds have a small protrusion on the end of their upper mandible as well, but unlike the nail of waterbirds, this small hard growth is on the topside of the beak and it disappears after the chick has hatched, usually in about a week. The protuberance is called an egg tooth and it is used by chicks to break through their surprisingly tough eggshells when hatching. Some species have an egg tooth on the lower mandible, which also disappears after hatching.
A bird’s mouth is a straight-forward affair. It is comprised of the upper and lower palate, the tongue, an opening to the trachea and another to the esophagus. The upper mandible has a slit down the middle which runs from the front of the beak to the back. This slit, called the choana, creates a cleft palate which can be found in all bird’s mouths. The back section of the upper palate has a slight depression which corresponds to the shape of the tongue, while the lower palate, lined with a soft mucous membrane, cradles the tongue.
The tongues of birds come in an amazing variety of shapes and sizes. The woodpecker is just one example, and its tongue, with respect to bill size, is the longest in the bird kingdom. In some woodpeckers, the tongue is four times longer than the beak. In order to accommodate this extreme length, the tongue actually curls around the outside of the woodpecker’s skull. The tip is coated with barbs that, along with the length, helps the woodpecker extract insects from crevices deep within trees.
The shape, size and length of birds’ tongues have evolved to help facilitate each species with obtaining and consuming food. Hummingbirds, for instance, have tongues with thin edges and a frayed tip that can be curled into a trough-like shape. Nectar is drawn into this trough by capillary action, then swallowed when the bird retracts its tongue back into its mouth.
Most species of birds have very muscular tongues, with the exception of the fish-eaters. Because birds like pelicans swallow their food whole, they have no need of a muscular tongue to help manipulate and guide their food as do the nut and seed eaters. The pelican uses its remarkably small tongue for very little except to cover the trachea when it dives for fish so that water is not inhaled.
A raptor’s tongue is triangular in shape with two rearward pointing projections which give it the look of a somewhat elongated arrow head. Those projections, along with a covering of tiny rough projections called papillae, help birds of prey hold and move food around. Most North American raptors have pink colored tongues, with the exception of the accipiters. The sharp-shinned hawk, Cooper’s hawk and goshawk, all have distinctive black tongues.
Both the trachea and esophagus have their respective openings in a bird’s mouth and for that reason, it is worth briefly covering their functions. The trachea, commonly referred to as the breathing tube, is part of the respiratory system. Birds bring oxygen into their lungs and air sacs by drawing in air through their nares or mouths into the trachea. The opening to the trachea is a simple slit-like aperture that is drawn closed when a bird eats or drinks. It is interesting to note that a bird’s voice does not originate in the larynx as ours does. Birds have a larynx, but, unlike ours it has no vocal chords. It is used primarily to regulate the amount of air coming into the trachea. Bird sounds are produced in an organ unique to birds called the syrinx. For most birds, the syrinx is located where the trachea joins the bronchi. Some Central and South American songbirds have their syrinx located at the lower end of the trachea, while some birds including certain species of owls have two syringes located in the bronchi. However, where the syrinx is located doesn’t seem to matter, for the real sound producers are the muscles that surround the syringeal membranes. As air passes over these membranes they begin to vibrate, and while they are vibrating, the surrounding muscles apply controlled tension which results in sounds of varying pitches - much like a violinist applying pressure to the vibrating strings of his instrument to create different notes. The turkey vulture is known for, among other things, its hissing voice. Actually, this New World vulture is limited to simple hisses because it has no syrinx at all.
 The slit-like opening of the trachea is found at the base of a raptor's tongue, just before the two rearward pointing projections that give the tongue the look of an elongated arrowhead. |
The esophagus also begins as a simple opening in the bird’s mouth and is the beginning of the digestive system. Through it, food passes from the mouth to the stomach. For some birds, however, food makes a stop in the crop before it reaches the stomach. The crop is a swelling at the base of the esophagus that forms a storage area where food can be held for later digestion. Having a crop is advantageous to seed eating species, like game birds, that are preyed upon. It allows them to quickly gather a substantial amount of food for later digestion, while limiting their exposure to predators. The crop is also used to soften seeds before digestion, and by pigeons and doves to produce a fat-rich food called "pigeon milk" with which they feed their squabs.
Of the raptors, only the diurnal birds of prey, the hawks, eagles and falcons have a crop. Like seed-eating birds, they use it to store pieces of food. An amazing amount of food can be stored in the crop which bulges out from the hawk’s upper chest as it fills, giving a fully cropped-up raptor the look of a feathered Mae West! Although powerful birds like hawks are not often thought of as vulnerable, they too are at risk when eating. Another raptor may steal their food, or, in the worst case scenario, a larger bird of prey or a mammalian predator may kill and, in turn, eat the raptor while it was occupied with eating. Being able to quickly swallow and store food lessens a raptor’s chance of having its kill stolen, or of it becoming the kill. The ferruginous hawk of the western U.S. shares its territory with the larger and more powerful golden eagle and, as a result, is a rather nervous eater. Golden eagles aren’t above stealing the hard-earned meal from the talons of a lesser raptor. Consequently, ferruginous hawks don’t waste time when dining. They consume their prey whole whenever possible or by tearing it into a few huge portions which are quickly bolted down. To facilitate this behavior, they have unusually large mouths. The gape of a ferruginous hawk is striking when compared to a red-tail’s, yet the birds are similar in size.
The storing of food in the crop also allows raptors to consume a greater quantity of food, thus providing them with needed nutrition at a later time. In addition to serving as a self-contained larder, a diurnal raptor’s crop functions as a way-station for indigestible items. Fur, feather, bones, scales, claws and the hard coverings of beetles are routinely eaten by raptors, but, they are undigestible and must be gotten rid of. These unusable bits and pieces are gathered in the crop where they are formed into a compact, elongated oval called a pellet or casting, and are regurgitated by mouth. Most raptors cast a pellet once-a-day, often right before they eat their next meal.
Owls also form pellets, although not in the crop because owls do not have crops. These nocturnal hunters capture and eat their prey whole. Food goes directly from the owl’s mouth to the stomach where any indigestible parts are formed into pellets. These pellets are much looser in consistency than the diurnal raptor’s, but are regurgitated by mouth as are the day hunter’s.
There are a few distinguishing beak or mouth features among raptors that are worth mentioning. One of the most unusual is the extremely slender, sharply curved beak of the Everglade snail kite. This medium-sized hawk exists almost exclusively on a diet of fresh water apple snails. According to the U.S. Fish and Wildlife Service, as of 1987, there were only five documented cases of Everglade snail kites preying on non-snail items. The snail kite is uniquely adapted to eating the apple snail which it deftly removes from the shell with its exaggeratedly curved beak. There is a price to be paid for exclusivity, however, and for this species it is the threat of extinction. The snail kite is endangered throughout its Florida range and its future doesn’t look promising. Habitat loss and destruction threaten it, as do indiscriminate shooting, excessive human disturbance and a loss of food supply due to pesticide and nutrient-laden runoff from adjacent lands. For this species, saving its habitat won’t be enough. If the little- thought-of apple snail ceases to exist, so too does the Everglade snail kite.
For sheer drama, the bald eagle’s beak is unequaled. Not only is it large, it also changes color. As the bald’s head and tail feathers change from brown to white, its beak is transforming as well. Over a four or five year period, as the eagle matures, its beak turns from black to yellow.
The nostrils, or nares, of the falcons are also notable because they have, what is generally believed to be, a special baffle within each nostril that regulates the amount of air entering the nasal cavity during high speed flights. This makes sense for the peregrine falcon that goes into a stoop of upwards of 200 mph when chasing avian prey, but, not all falcons fly at great rates of speed. The North American kestrel, for instance, hovers above fields like a tiny helicopter to spot prey, then drops on it for the kill. Dr. Tom Cade, in his book Falcons Of The World, suggests that this central bony tubercle, which is an extension of the septum, may be used in an as yet undetermined way in olfaction, or that it could function to indicate air speed by sensing changes in pressure or temperature produced by differing external air-stream velocities, in much the way that Mangold (1946) suggested the valve-like pockets in the nasal chambers of the fulmars, shearwaters and albatrosses do. His suggestions are compelling because as he points out, there is no experimental evidence to support the wind baffle theory, and it seems unlikely in view of the fact that eagles and other raptors, which have large nares without these tubercles, also stoop vertically at high speeds, while the slow, largely cursorial savannah hawk of South America has independently evolved a very similar structure in its nares.
Raptors have a salt-excreting gland in their nares, located in the nasal orbits, that can secrete sodium and chloride ions to help regulate these electrolytes in their blood and tissue fluids. Falcons sneeze out these nasal gland secretions, while other raptors allow the fluid to flow from the nares to the end of the beak where it is then flicked off with repeated head shakes. When feeding their young, a salty fluid also flows from a raptor’s nares to the tip of the beak where it falls on the chick’s pieces of food. It is not yet known if this fluid has any nutritional value for the young.
Each member of the falcon family from the small 5 oz. kestrel to the large 4 lb. gyrfalcon have a beak with the same distinctive feature, a tomial tooth. The tomial tooth is located on the outer-edge of the upper mandible near the curved part of the beak and is a triangular-shaped downward pointing projection that fits into a corresponding notch on the lower mandible. With this tomial tooth falcons are able to quickly dispatch their prey by biting through the neck vertebrae and severing the spinal column. The only other bird possessing a tomial tooth is the shrike, a predatory passerine often called the butcher bird.
There are many more remarkable facts about bird’s beaks that I haven’t covered, such as the beak of the tropical fruit pigeon that opens both vertically and horizontally to accommodate the swallowing of fruit pits the size of a small hen’s egg, or the beak of the 2 oz. hawfinch that can exert a crushing force to an olive pit of 106 to 159 lbs! Yet, mention the word bird and we still think wings. Perhaps we should be thinking beak instead, for beaks, in all their wondrous shapes, sizes and functions, define the bird in a way wings can’t equal.
The Eyes Have
It
(Raptor eyes that is)
by Stephanie Streeter
from DVRC Journal Fall/Winter 2001
Being characterized as eagle-eyed implies having superior vision, but, even at our very best we humans can’t begin to compete with the birds. They have the best vision of any animal on earth, and the raptors just might be the avian champs. A golden eagle can easily spot a jackrabbit a mile away. What makes this visual feat so impressive is not the distance alone, but how well the eagle sees at that distance. This is called visual acuity and because of the eagle’s amazing visual acuity it can not only see and identify that jackrabbit a mile away, it can also watch the rabbit’s sides expand and contract with each breath. From a quarter-mile away, it can see the blink of the animal’s eyes, the delicate twitch of its nose, and distinguish the individual hairs that make up its quivering whiskers. To put this in perspective, if our eagle could read and someone held up a New York Times a quarter-mile away, the eagle would know what the day’s headlines were. We, on the other hand, would just barely be able to make out some fool standing in the middle of the desert with a big white obstruction in front of his/her face.
So, just how and why do birds see so well? Part of that answer can be found by just looking at a bird. Birds have huge eyes, particularly owls. In the Life of Birds by Joel Welty, he states that, “although the weight of a man’s head and the weight of a starling’s head are both about one-tenth of their overall body weight, the ratio of eye weight to head weight in the man is less than 1%, while the starlings is about 15% (Pumphrey, 1961)." For owls this percentage is even higher and, in fact, some owls have eyes larger than a human’s. The advantage to this is that birds can see larger, sharper images which is handy whether you’re the predator or the prey. But, in order to have large eyes, sacrifices must be made. In the case of the owls this is easy to see once a closer look is taken inside the skull. After getting rid of all the head feathers that deceptively give owls their big-headed appearance, a one and one-half pound barred owl is left with a skull slightly larger than a golf ball. It also has eyes the size of a human’s. Now, imagine if you will, opening up that golf ball and stuffing two human-sized eyes into it. Once closed, what you will have is a ball with very little space to spare. But wait, we aren’t quite done yet because into the remaining space I want you to squeeze a brain. And there, of course, is where the owls’ sacrifice is made. They have small brains. But compensations have been made. In addition to a remarkable sense of sight, owls have an uncanny sense of hearing, and they are silent in flight. These night hunters are so well equipped for survival they don’t need much in the way of cognitive skills.
Structurally, a raptor’s eyes are very much like ours, but only the colored part of the eye (the iris) and the pupil are visible. The whites of the eye, or the sclera, are hidden behind the bird’s eyelids which masks just how large their eyes truly are. To see clearly, diurnal birds of prey, with eyes placed on either side of the head, must turn their heads in order to have forward binocular vision. Their eyes are pretty much fixed in the sockets - only a small amount of lateral movement can be made - but they are able to turn their heads further than we can and with greater ease because of extra neck vertebrae. While all mammals, including humans, have 7 cervical or neck vertebrae, birds, depending upon the species, have from 8 to 25 (the 25 being laid claim to by swans), with 14 being the average. This incredible flexibility has given rise to some enduring old wives' tales such as an owl being able to rotate its head in a complete circle, or if you walk ’round and ’round a sitting owl it will follow you till it screws off its head! Neither is true, of course, although owls can rotate their heads in an impressive arc greater than 180 degrees in one direction. Some references credit them with being able to turn their heads 270 degrees; but, after 20 plus years of working with these birds, I have yet to see them exhibit that amount of cervical dexterity. In fact, even after death, I cannot turn an owl’s head much past 180 degrees without risk of breaking the neck.
 With beak raised, the bittern assumes a position of camouflage among surrounding reeds yet is still able to watch for predators because of the extreme forward position of its eyes. (Illustration by Julie Collier based on a photograph by Richard D. Robinson.) |
Birds possess both binocular vision - when both eyes focus on a single object and monocular vision - focusing on an object through only one eye. Where a bird’s eyes are placed on its head determines the range of its binocular and monocular vision. The woodcock, with eyes situated far on the top of its head, has binocular vision both forward and backward, while the bittern, which has its eyes located low on the head, has binocular vision straight forward when its bill is pointed upward - useful for keeping an eye on the enemy when your means of camouflage is throwing your bill straight into the air in imitation of surrounding reeds. As for the birds of prey, they have a field of binocular vision between 35 to 50 degrees of arc, and monocular vision of approximately 150 degrees depending upon the species. The blind spot, where they cannot see without turning their heads is about 20 degrees. Owls, with their forward facing eyes, have binocular vision of 60 to 70 degrees and about the same range of monocular vision.
All birds have eyelids, but they’ve gone us one better. In addition to moveable upper and lower lids, birds also have a third eyelid know as the nictitating membrane or nictitan. This is a semi-transparent membrane (although in owls alone it is opaque) that can be swept closed across the eye from the direction of the beak to the ear very quickly at the bird’s will. The nictitating membrane is used to protect the eye as well as clean and lubricate it by brushing moisture secreted from the lachrymal gland across it with each blink. Just prior to striking their prey, raptors close their nictitans for protection against the possible lashing out of the captured animal. A peregrine falcon in its 200-mile-per-hour stoop draws the nictitans across its eyes as protection against the wind, while ospreys use them as built in water-goggles when they plunge into the water feet-first for a fish. Diving ducks, loons and auks have an added visual aid because located in the center of their nictitating membranes is a clear lens-shaped window of high refractive index that serves as a “contact lens” when they are underwater (Walls, 1942).
Unsure of what will befall her, this golden eagle draws
the nictitating membrane across her eye while being fitted with a transmitter.
While the nictitating membrane is used for blinking, the true eyelids are reserved for sleeping in most avian species. But, there are exceptions. Pigeons use all three eyelids to blink, while owls use their upper lids to blink - making them appear to be saucily winking on occasion - and their lower lids to close their eyes for sleep. Birds alone, of all the animal species, including humans, close their eyes in death.
Given that raptor eyes function in the same basic manner as ours (like a camera with light entering the pupil, passing through the lens and casting a visual image on the film, or in the case of the eye, the retina) and have many of the same parts - lens, cornea, pupil, iris, retina, vitreous humor, optic nerve, etc. - why, then, are birds of prey able to see so much better than we, or any other animal on earth? One of the reasons is their great power of accommodation (focusing). The focusing power of an eye is measured in diopters which is the reciprocal of the focal length of a lens in meters. What this means is that a lens of one diopter will focus on an object one meter away (and beyond), while a lens of two diopters will focus at a distance of ½ meter away (and beyond). A child has a focusing power of 13.5 diopters, but by the time he/she is 40 it will only be about 6 diopters. In contrast, a cormorant, and other water birds, have an outstanding focusing power of 40 to 50 diopters. Most land birds have an accommodation between 8 and 12 diopters, while owls and other nocturnal birds measure in with a 2 to 4 diopter accommodation. Owls cannot focus their eyes on close objects which is why it is relatively easy to pick up a perched saw-whet owl by distracting its attention with one outstretched hand while coming up close to its body with the other and grabbing it.
Birds that need to change the focus of their eyes from near-sighted to far-sighted very rapidly, like a dove being chased through the woods by a goshawk, have soft, malleable lenses. The shape of these lenses, and thus their power of accommodation, is changed by means of pressure being applied to the lens through the Brucke’s muscle of the ciliary body acting on the sclerotic ring and annular pad. Very simply, muscles on either side push the soft lens into a more curved shape for close focusing and relax letting the lens flatten for distance focusing. Meanwhile, the goshawk pursuing our beleaguered dove can change its eye focus just as quickly, but it does so by a uniquely different means. Hawks and owls focus their eyes by changing the curvature of the cornea with a ring of ciliary muscles known as Crampton’s muscles, in addition to changing the surface shape of the lens. Like the dove’s eye, the human eye also changes focus by changing just the lens’s shape. But, instead of direct pressure to the lens, a stretching tension on the lens released by the action of ciliary muscles allows the elasticity of the lens itself to determine its shape. While effective, it does not allow us to change from near-sighted to far-sighted or back again as quickly as birds do, nor do we see as acutely once our eyes have focused.
A bird’s extraordinary visual acuity can be explained by taking a closer look at the retina. This is where images are formed and a bird’s retina is almost twice as thick as a human’s. It is densely packed with visual cells (rods and cones) and has a high concentration of nerve fibers in those cells. The number and size of the cones determines, in large part, the resolving power of the retina. All diurnal birds have more cones than rods and these cones can be found sparsely scattered throughout the retina, with a high concentration in the fovea. The fovea is where the sharpest vision occurs for both man and bird. According to Welty that is because, “it is here where overlying nervous tissue is thinned away and the visual cells are packed together in a funnel-shaped pit which allows for the highest resolution.” One of the things that helps to make all diurnal birds of prey “eagle-eyed” is having a second fovea, the temporal fovea. It gives them sharp binocular vision, while the central fovea provides sharp monocular vision. Used together they can triangulate their field of vision and pinpoint the exact location and distance to their prey. Other birds, especially those that feed on the wing like swallows, also have a temporal fovea.
What helps give diurnal raptors the bragging rights to best visual acuity - the ability of the eye to focus clearly and sharply on an object as it becomes smaller or more distant - is the number of visual cells in each fovea. We humans have 200,000 visual cells per square millimeter in our fovea. Not bad until you consider that the small, unassuming English sparrow has 400,000 per square millimeter and the European Buzzard, the visual cell grand champ, has one million per square millimeter giving it a visual acuity of at least eight times that of a human. Remember our newspaper reading eagle at the beginning of the article? Now you know why it can see the headlines while we see only a white rectangular shaped blur - two foveas per eye, each densely packed with visual cells.
 Strange head contortions like this golden eagle's are executed by diurnal raptors to improve their overhead vision. |
An interesting fact about diurnal birds of prey, which explains why at times they appear to be engaged in painful head and neck contortions when perched, is that they have more sensory cells in the upper hemisphere of their eyes (which perceives images from the ground) than in the lower hemisphere (which views the sky). This causes a hawk to invert its head either near its belly or over its back when it wants a better overhead view.
Hawks and eagles, as well as many birds of open country, like swallows, terns, ducks and shorebirds have an added visual refinement. Across each retina is a horizontal sensitive streak, usually with a fovea at each end. This sensitive streak is parallel to the ground when the bird holds its head in a normal position, and allows it to economically and sharply scan the horizon without any head or eye movement.
That birds see well is indisputable, but do they see in color or black and white? Like us, all diurnal birds see in color because the cones that give them such keen eye-sight also allow them to see colors. Even without scientific verification, simple observation should tell us that a bird’s world is a colorful one. The plumage of most males is bright and varicolored - for some, such as the golden pheasant and certain birds of paradise, it approaches downright gaudy. And even those that aren’t as brilliantly colored have plumage that is brightest at the beginning of the mating season (compare a spring gold finch to a fall one). Color is an integral part of many diurnal species mating rituals, otherwise it would be so much wasted effort for the peacock to unfurl its flamboyant tail, the blue-footed booby to wave around its outlandish feet, or the frigate bird to inflate its red throat pouch. From the beginning of a bird’s life, color is important. Nestling songbirds gape, displaying intensely colored mouths that induce parental feeding, while young gulls peck at a distinctive red dot on the underside of the parent’s beak causing them to regurgitate food.
Birds have another tool in their bag of visual tricks. Many of the cones in their eyes contain colored oil droplets (one droplet per cone) that help to sharpen their vision. The diurnal birds’ oil droplets are red, green, orange, yellow or colorless, while crepuscular and nocturnal species’ are mostly colorless or pale yellow. These oil droplets enhance vision in two ways. First by heightening the contrast of colored objects in the field of view. Welty, in The Life Of Birds states, “Most birds eat small objects such as seeds, berries and insects which are often colored unlike their immediate surroundings. If a red insect in green foliage is scanned by a bird having red oil droplets in its cone cells, the insect will “blink on and off” like a single blinking light bulb amid hundreds of steadily glowing bulbs as its image sweeps across cones that do or do not possess red oil droplets. This is because a red filter allows red light to pass through it while hindering other wave lengths.” Secondly, the oil droplets may allow birds to see better during hazy weather by acting as a filter that holds back some of the glaring short wave lengths of light while permitting more of the longer waves to stimulate the retina.
Perhaps the most remarkable refinement to avian vision is the ability by some species to see in the ultraviolet light range. That the European kestrel, and probably other raptors as well, have this ability was proved by Finnish scientists, Viitala, Korpimaki, Palokangas, and Koivula. Being able to see ultraviolet light allows the kestrel to track prey, such as mice and voles, that mark their trails with urine and feces which absorb ultraviolet light. For falcons in treeless areas (such as farmlands) this means they can rapidly scout large areas and spot rodents by simply following the ultraviolet trails that delineate their movements. It is also thought that black-chinned hummingbirds can see in the ultraviolet light range, allowing them to detect nectar bearing flowers that strongly reflect ultraviolet light.
That some species can see in the ultraviolet light spectrum has been proven, but what about in very low light? This is where the number of rods in a bird’s eye becomes important, for they are what allow nocturnal birds, such as owls to go about their business in the dead of night. Because an owl’s eyes are rich in rods and sparse in cones, they have poor retinal acuity and limited accommodation. And, because cones are responsible for color discrimination, their world is probably one composed in black, white and gray, with a very limited amount of color. It is not surprising to learn that owls, such as the burrowing owl, that are active during the day, have more cones in their retinas and are able to see more colors than their nocturnal cousins. But before feeling sorry for the night hunters, consider how and what they can see. Owls, who see as well as we do during the day, have large, light gathering pupils to help them see even better at night. Light collected by the pupil is sent to the rods, which contain a purple-red pigment called rhodopsin. Rhodopsin, or visual purple, is extremely sensitive to even the smallest amounts of light, allowing owls, with their rod-rich eyes to see at night. And just how well do they see in the dark? They cannot see in total darkness, but, add just the tiniest bit of light and an owl can go about its business as easily we do during the day. According to Julio De La Torre, in Owls, Their Life And Behavior, “Long ago, L.R. Dice showed that long-eared owls could see dead prey (freshly killed mice) up to six feet away at a level of illumination of 0.00000073 foot candles. (A foot candle is the illumination produced by a standard candle at a distance of one foot.) More recently, J. Lindblad has shown that long-eareds do even better than that: they can find dead prey at ten feet at 0.00000016 foot candles.” To illustrate just how remarkable this is, Torres suggests that a long-eared owl trapped in the Astrodome would probably be able to find a stationary mouse with just one candle lit in the center of the arena, and, of course, would not fly into any posts or obstructions while doing so. I particularly like his summation of an owl’s capabilities - “combine the owl’s extraordinary, synchronous eye-ear balance with silent flight and razor-sharp claws and you have a passport to eternity for foolhardy mice.” There is no need for us to feel sorry for the cone-impoverished owl.
Have you ever wondered why pigeons thrust their heads forward when walking, why falcons bob their’s up and down or owls swivel their’s? These seemingly erratic head movements have to do with vision and, in the case of the pigeon and falcon, is known as “parallactic” location or optical fixation. The pigeon, by moving its head forward, compensates for the motion of its body so that its head settles into a series of fixed positions. This allows it to more easily spot anything moving within its field of vision, such as an attacking falcon. As for our head bobbing falcon, by rapidly raising and lowering its head, it shifts the relative position of the object (such as a tasty pigeon) it is viewing which facilitates its ability to judge the object’s distance. For an owl, no matter what sort of head movement it makes, oscillating horizontally or vertically, or rotating, it is done to reinforce depth perception.
There is no arguing that raptors are visual creatures. Large eye size, greater focusing power, and a much larger number of visual cells insure that they possess the keenest eyesight on earth. Falconers knew this thousands of years ago which is why to calm their hunting hawks they hooded them. I too know this, which is why each time my husband’s 22 year-old red-tailed hawk, Mariah, looks to the sky and starts screaming her territorial cry, I look up as well. I know that eventually a spot will enter my field of vision, and that if I am patient enough, it will slowly resolve into a red-tail on the wing. She’s never been wrong.
References: The following publications were used to write this article. Anyone wishing to learn more about raptors, avian vision, or birds in general are urged to consult them.Brown, L. 1997. Birds of Prey. Chancellor Press, London, England
Cruickshank, A. D., Cruickshank, H. G. 1976. 1001 Questions Answered About Birds. Dover Publications, Inc., New YorkDe La Torre, J. 1990. Owls, Their Life and Behavior. Crown Publishers, Inc., New York
Grossman, M. L., Hamlet, J. 1964. Birds of Prey Of The World. Bonanza Books, a division of Crown Publishers, Inc., New York
Hamlyn, P. 1965. The Pictorial Encyclopedia of Birds. Hamlyn Publishing Group Limited, London Kochan, J. B. 1995. Birds Heads & Eyes. Stackpole Books, Mechanicsburg, PA
Page, J., Morton, E. S. 1989. Lords Of The Air The Smithsonian Book Of Birds. Crown Publishers, Inc., New York
Peterson, R. T., Editors of Life Magazine. 1963. The Birds. Life Nature Library. Time, Inc., New York
Pumphrey, R.J. 1961. Sensory organs: hearing. In Marshall, A.J. (ed). Biology and Comparative Physiology of Birds. Academic Press, New York
Walls, G. L. 1942. The Vertebrate Eye and Its Adaptive Radiation. Cranbrook Institute of Science, Bloomfield Hills, Michigan
Welty, J. C. 1979. The Life of Birds. Saunders College Publishing/Holt, Rinehart and Winston, Philadelphia
Viitala J., Korpimaki E., Palokangas P.& Koivula M. 1995. Attraction of kestrels to vole scent marks visible in ultraviolet light. Nature 373:425-427
What’s In A Wing
by Julie Collier
from DVRC Journal Summer/Fall 1989
Remember the Greek myth of Daedalus
and his not-very-bright son Icarus? Daedalus was supposedly a brilliant
sculptor, architect, and inventor, one of those tiresome all-rounders who could
do anything he chose to. He is credited, among other things, with inventing the
axe, the wedge, the gimlet (the tool, not the drink), and the sailboat. In his
spare time Daedalus built the Labyrinth for King Minos of Crete. Minos, a real
ingrate, accused Daedalus of treason and imprisoned him and Icarus in the
Labyrinth. To escape from its maze of passages, Daedalus fashioned large wings
of feathers held together with wax for himself and his son. Off they flew, the
story tells us, Daedalus very sensibly to Sicily, where presumably he went on
inventing useful items. Icarus, who marched (or flew) to a different drummer,
opted to fly toward the sun. The wax that held his wings together melted in the
sun's heat, the wings fell off, and the twit fell into the sea far below. The
island on which his body washed up was later named Icaria.
Well, it's a good story - one that has potential as a TV movie - but like all myths the story of Daedalus and Icarus is more than just a story. It allegorizes, explains, and we can guess that the character of Icarus is meant to represent presumption, delusions of grandeur, and the dangers of not following parental example. Man's place, the myth is telling us, is not in the air.
It's certainly true that we have no business in the air sporting wings like those Daedalus invented. Fortunately this gentlemen's (mythical) reputation as a brilliant inventor doesn't rest solely on his Labyrinth-leaping wing design. If it did, I'm afraid Daedalus would have to be considered a washout, because no human could fly successfully with those feather-and-wax contraptions he reportedly devised. Other inventors through the centuries have tried literally to take wing, to lift themselves into the air using winglike constructions patterned after birds. And this concept has never worked. The structure of the human body simply isn't designed to meet the demands of this type of flight, and no amount of training can overcome this deficiency. We have achieved flight only by coming up with artificial devices.
These artificial devices, whether we call them gliders, powered aircraft, or rockets, are not really structured like birds. True enough, the principles of aerodynamics ("air motion"), the physical laws that allow a heavier-than-air object to move through that air, govern both birds and our mechanical inventions. But birds and man-made flying machines have adopted different means of using those laws to advantage.
(People first learned what it was like to be airborne when the Montgolfier brothers discovered that hot air rises because it is lighter than cool air, and capitalized on this principle by making the first balloon ascent in 1783. But hot-air balloons and the later airships -dirigibles, zeppelins, and blimps - failed to utilize the really crucial elements of bird flight such as aerodynamic lift, speed (increased by streamlining), and exploitation of the downward pull of gravity. Balloon flight, based on the lighter-than-air principle, proved to be a dead end in aeronautics.)
Oddly enough, then, our attempts to translate bird flight into human flight may have slowed our progress in developing successful aircraft. Certainly the longing to fly has been with us for millennia, and it crops up in myths from many cultures. Almost unanimously, religions have described divine beings as winged creatures. Egyptian gods and goddesses were shown with wings that may have been patterned on the long, magnificent wings of vultures. The Hebrews visualized seraphim and cherubim as winged. Since early rulers were often thought of as divine, legends began to circulate of their airborne exploits. A Chinese emperor of the Han dynasty reputedly kept an eye on his empire from a flying chariot. The Persian king Keykavus took this l-spy concept one step further - he oversaw his lands from the comfort of his throne, which was transported by four apparently very muscular, eagles.
Some thinkers did give serious consideration to the question of human flight. Leonardo da Vinci sketched a variety of wing-flapping devices in the sixteenth century that were based on his observations of birds in flight (he even came up with a design for a helicopter-like machine). His drawings are remarkable, for they show that this prototypal Renaissance man understood certain principles of bird flight that have only recently been explained. Clearly da Vinci had inferred the idea of lift from his bird-watching, because he used it to explain complex aerial maneuvers. The trouble was that da Vinci, like many before and after him, based his ideas for flying machines on the analogy of bird flight. And this analogy can be applied only to soaring and gliding devices. A wing-flapping machine simply cannot power a person through the air in the manner of birds.
Leonardo de Vinci's sketch of an
arm-and-leg-powered ornithopter.
At least one man foresaw that a fixed-wing type of aircraft might one day be feasible. In the seventeenth century John Wilkins, who was both lord bishop of Chester and one of the founders of England's scientific Royal Society, thought out four ways in which human flight could be achieved: (1) with the spirits of angels: (2) with bird power; (3) with wings, a la Daedalus and Icarus; and (4) in a flying chariot. Wilkins may have been a visionary but he was no fool, and he was not hopeful about the practical applications of the first three ideas. However, he decided that the flying-chariot concept had possibilities. He wrote: "If fowl can so easily move itself up and down in the air without so much as stirring the wings...it is not improbable that when all due proportions of [a feasible apparatus] are found out, and when men by long practice have arrived to any skill and experience, they will...come very near unto the imitation of Nature."
That was just the trouble, though - man continued to imitate nature in his quest to conquer the air, and it didn't work. A wing-flapping flying machine is known as an ornithopter ("bird-wing"), and although many inventors have tinkered with the concept, this idea of flapping flight is another dead end in the history of aviation. The Wright brothers ushered in the era of aviation by coming up with the world's first practical, powered, fixed-wing airplane, and by flying it successfully in 1903. The Wrights' history-making plane, and all the aircraft that have evolved from it up to the present day, do not fly like birds. The question is, if wing-flapping flight didn't work for us, why does it work for birds? Just how do bird wings work, anyway?
One very good reason that wing-flapping flight works for birds and not for us is the fact that birds are designed to fly, and we are not. They are born with the capacity for flight, do not need to learn how to fly, and we are not. They have evolved over millennia as specialized flying beings, and we have evolved as generalized earthbound beings. What special structures have birds evolved for flight? Besides feathered wings, birds have hollow bones for lightness. The only birds that do not have these wonderfully light yet strong bones are flightless birds like the ostrich, and deep divers that need dense bones for staying underwater.
Let's take a close-up look at these feathered wings that work so well for birds. Unlike fixed-wing man-made aircraft, which require a power source, bird wings both power the bird and, by their shape and construction, provide the efficient aerodynamics that make flight possible. Where does this power come from? How do the wings move? Three muscles provide most of the force necessary to move a bird's wings. These three are the large and small pectoral muscles, and the deltoid muscle. When the large pectoral muscle is contracted by a bird, the wing is pulled down. The small pectoral muscle and the deltoid muscle are responsible for moving it upward. Since the large pectoral muscle does most of the work of moving the wings, it is the largest muscle a bird possesses. When we eat chicken or turkey breasts, that's what we're consuming. The pectoral muscles are attached to the breastbone, which in birds is huge, with a deep keel that can be felt under the bird's skin. The furcula, which we remember from childhood as the "wish-bone," is located just above the breastbone.
The wings powered by these muscles are covered by feathers. The outer flight feathers are called primaries, and these are inserted in the bone of the wing's "hand." Most birds of prey have ten primaries, and usually the feathers are markedly pointed, as well as being asymmetrical, with a narrow outer vane. The inner flight feathers, located along the bird's "arm," are attached to the bone of the ulna, and are called secondaries. They are usually rounded and symmetrical, and they vary in number. Song-birds, for example, have only nine or ten, while albatrosses, which stay aloft for long periods of time, have 37. As you might gather from that, the secondary feathers provide lift (we'll get to lift in a moment), and it has been shown experimentally that if half of a bird's secondaries are removed, the bird can still maintain flight, but shows some control problems. Covert feathers cover the wing, not only insulating and protecting it, but giving it its aerodynamically correct profile as well. The tail feathers are also important in flight; they function as direction-changers and as brakes. All the feathers work independently of one another, and allow the shape of the wing to be changed somewhat as the bird encounters different flying conditions.
The feathers themselves are changed annually. Birds molt at intervals of roughly 12 months, replacing old, broken, or worn-out feathers with new ones. Feathers are made of keratin, the same substance that horns, hooves, and fingernails are composed of. Keratin is strong, but feathers are subjected to constant friction as they move through the air, or against each other as the wings are folded and unfolded. So replacement is necessary, and is carried out during the least stressful time for a bird - in the summer, when breeding is over and food is plentiful. The feathers are dropped in a species-specific sequence, and matching feathers from each side of the bird will drop simultaneously. In birds like raptors that must fly in order to find food, the feathers are dropped gradually, so the bird is never without the powers of flight.
As you might expect, the basic shape of bird wings varies somewhat depending upon the species' lifestyle. There are four basic wing shapes. Gamebirds such as grouse and pheasant, as well as groups like the woodpeckers and the crows, have elliptical wings, rather short and rounded in proportion to the bird. The upper surface of this type of wing is highly cambered, or curved, and the outermost primary feathers of each wing are slotted. This shape helps its owner to dodge adroitly through brush and trees, but is not efficient for sustained high speeds. Many raptors, by contrast, have evolved a second type of wing - long, relatively narrow, without much camber, and tapering to a point (falcons, fastest of the raptors, show this taper particularly well). Many raptor wings sweep backward in flight, like those of a jet fighter. Nor are raptors alone in this design for high speed. All bird speedsters are built the same way, from swallows and swifts to waders and waterfowl.
A third type of wing is found among birds such as gulls that glide a great deal, and for long distances. This type of wing is also long and narrow, but it is flat as well, lacking the high camber or arch seen in the gamebird wing. The primary feathers of this third wing type are not slotted. The fourth and last type of wing has been evolved by soaring or gliding birds such as vultures and eagles. In this type the wing is broad, so the bird is well supported, and it is relatively shorter than a gull's wing, so its owner can make use of every little change in the air current. The primary feathers of this wing type are slotted, meaning that they can be moved as separate airfoils. This wing provides a maximum of lift, so vultures and eagles can hang in the air for extended periods, gracefully riding the thermals and updrafts without needing to flap their wings.
While bird wing shapes do vary according to the needs of the different species, there is one thing all bird wings have in common. They all have wings that are convex, to a greater or lesser degree, on their upper surface. Why? Well, the next time you're a passenger in a moving car, put your hand out the window, cup it slightly so it's upwardly convex like a bird's wing, and hold it in place. If the car is proceeding along at a good clip, you can feel your hand being moved more strongly upward than backward. This force is called lift, and it is lift that supports both a bird and an airplane in flight. What does an upwardly convex surface have to do with lift? When a wing, with its cambered or convex upper surface, is tipped at a slight angle to the air flow, the air current moves faster over this curved upper surface than it does over the lower surface. This creates a loss in air pressure above the wing, and so lift is created. Simultaneously, however, another force is at work. At the same time that the wing is being lifted by the difference in air pressure, it is being pulled backward because of its resistance to the moving air. This backward pull is called drag, and the two forces, lift and drag, are responsible for lifting the wing and pulling it backward. The overall shape of a bird wing has evolved to acquire as much lift as possible with as little drag as necessary.
All right, so now we have an upwardly convex wing that responds to the forces of lift and drag. What happens when this wing is flapped in the process of flying? Two separate movements have to be looked at here. The first is the power stroke, the downstroke that moves the wing forward and down. The second is the backstroke, or upstroke, that returns the wing to its original position, so it is ready for the next power stroke. While these two movements are taking place, the primary and secondary feathers play different roles. The outermost feathers, the primaries, pull the bird forward. The inner feathers, the secondaries, provide lift. During a power stroke, the primary feathers are not slotted, or separated, but are kept together to form a single unit. This produces an efficient airfoil that gives maximum thrust with a minimum of drag. Smaller birds separate the primaries on the upstroke, which cuts down on drag by allowing air to move through the feathers freely. Birds with larger wings have found this method impractical; they flex or even partly close their wings on the upstroke.
While all sizes and species of birds flap their wings in pretty much the same way, some are more efficient at flapping than others. Why? Because birds vary in wing loading, in their wing area divided by their weight. Birds with large, broad wings and relatively low body weights, like owls, show low wing loading. Falcons, on the other hand, have relatively high body weights for their wing area, and so show high wing loading.
We've seen what happens during flapping flight. What happens when a bird chooses to glide without flapping its wings? Gliding is possible only when a bird can manage to equalize its body weight with the forces of lift and drag. The force of gravity is used by gliding birds as well. Let us say that a vulture is airborne over the African plains. Its long, broad wings are extended to their fullest. Since the bird is not flapping those wings, it is slowly losing height because the force of gravity is constantly pulling on it. However, gravitational pull has its advantages; it provides speed. The vulture adjusts its wings so that its speed will propel it forward, even as it is losing altitude. Heavy birds with a small wing area will lose altitude more quickly than light birds with a large wing area. Vultures, therefore, are more efficient gliders, and can remain aloft longer, than birds such as falcons.
A gliding vulture being pulled
earthward by gravity must, sooner or later, flap its wings, crash, or find a new
air current to support it. Over land there are two main types of upward-moving
air for birds to utilize. The first is a thermal, which is formed when air is
heated by the warming surface of the earth as the sun gains in power in the
morning, or even by man's activities in towns and cities. The warm air rises to
be replaced by cooler air moving downward from higher areas of the atmosphere.
Thermals are certainly found in temperate climates - our raptors ride them here
in the Northeast - but they are particularly prevalent, and fast-forming, in the
tropics, where the land is warmed quickly by a hot sun in the morning. The
vultures that are an indispensable part of the African fauna ride these thermals
for hours. The birds can travel vast distances without so much as a wing flap by
shifting from a rising thermal to a layer of cooler air, riding the cool air in
spirals downward, and then picking up a new thermal and rising with that
one.
The second kind of upward-moving air used by gliding or soaring birds is an updraft or obstruction current. This type of current is formed when moving air encounters a solid barrier, such as a mountain. When it hits the obstacle, the air is forced upward. Migrating birds follow mountain chains in order to utilize the updrafts that will hold them aloft with a minimum expenditure of energy. Seabirds can often be seen soaring above cliffs when they have the advantage of an onshore breeze.
We've talked about the role of bird
wings in flapping flight and in gliding or soaring. The last type of flight
we're going to look at is hovering, and it's not a skill that all birds can
manage, at least not efficiently. Two birds in the Northeast that hover
routinely, and who are good at it, are the American kestrel and the
ruby-throated hummingbird. The hummingbird, however, is the true master of
hovering, and its body is designed to help it in its mastery of t
his specialized form of flight. Hummingbirds
can not only hang stationary in the air, but can move forward or back, left or
right, up or down, even upside down -something no other bird can manage. To
accomplish these unique feats, the hummingbird boasts huge flight muscles for
its size. The two pectoral muscles and the deltoid muscle comprise 30% of its
body weight. However, as you might expect, its wings have evolved into a shape
that differs from that of other birds. The hummingbird has exceptionally long
"hands" that hold ten primary feathers, while the "arm" area is proportionately
very short. A hummingbird has only six or seven secondary feathers attached to
this "arm," while the soaring albatross may
have as many as 40. The hummingbird also has a shoulder joint so moveable that
it can turn in all directions. The moveable shoulder joint and the
well-developed wing tips give the hummingbird
an unparalleled amount of control over its wings, so it can perform aerial
maneuvers other birds cannot. The kestrel, for example, despite its nickname of
"windhover," is not the hovering master that the hummingbird is. The kestrel's
wings are not specialized for hovering, and it cannot hover in still air as the
hummingbird can. The kestrel must have an air current to help support it,
although it is able to utilize even the slightest breeze to the fullest
extent.
Birds and flight are completely intertwined. A bird looks like a bird, acts like a bird, is a bird, because of its design for flight. Because birds are, in the main at least, safe from all but aerial predators, they have been able to evolve a complicated language, and one which they don't hesitate to use. They habitually communicate with each other, and proclaim hunting and nesting territories, to everything in earshot. Birds are, to put it bluntly, downright loud creatures, noisy in a way that mammals are not. Think of a spring day outside. What do you hear? Most of the natural sounds you pick up are bird songs or calls. Birds can afford to be that obvious because of their ability to fly. This gives then a measure of safety denied other animals. In the same way, birds sport a staggering variety of colors and shades in their plumage, unlike the more somberly and uniformly dressed mammals. The colors are possible because birds have much less need for concealment than other creatures. Much of the charm, much of what we admire in birds, is part and parcel of their design for flight.