revised March 11, 2014
Adaptations for Bird Flight
The first animals to develop powered flight were prehistoric insects. Their wings are not at all like those of vertebrate flyers, either bird or bat, and their flight patterns, even today, are also quite different. Bird and bat wings replace front limbs. Insects retain all their legs, and their wings most commonly develop from second and third thoracic segments – in some, wing muscles are directly attached to wing base; in others, wing muscles attach to the thorax and deform it.
The second animals, and the first vertebrates, to develop flight were reptiles – dinosaurs, or to be more exact, pterosaurs. Birds evolved from dinosaurs, as we have good evidence today, but probably not from the pterosaurs. Most ornithologists and paleontologists believe birds arose from the dinosaur group called theropods, who were probably not flighted themselves (hint: the name means “beast foot”), but may have developed a wishbone and feathers. These theropod fossils have so far evinced no flight-feather traces. Today’s flighted birds have off-set vanes (the central stems) in wing and tail feathers, necessary for aerodynamics and feather control during powered flight. The feathers on dinosaurs were very likely part of their display equipment and thermoregulation (controlling body temperature), roles feathers also play in birds today in addition to making bird flight possible.
Many species of pterosaurs developed full flight, some quite large – one fossil species reveals a wing span of about 35 feet or more. To see reconstructions of these strange and marvelous flyers, Google “pterosaurs,” and click on a link to Bob Straus’s pictures – fabulous!
Whether a huge species, as in the picture above, or one as small as today’s bats, pterosaurs’ wing webs stretched from the elongated fourth digit of their “hands” to their feet, and most had a second web connecting their legs. Some had long tails. How the larger pterosaurs flew, with such huge wings and great weight, is a question still being explored. But researchers have long known that they were quadrupeds, not bipeds like birds, folding their wing “finger” up and back and walking, using their “hands.” They had powerful muscles in their front limbs, as opposed to modern birds (whose flight muscles are all centered on the sternum), and apparently launched themselves using both front and rear limbs. (In the illustration of comparative front limb bones, above, note the position of the hand and fingers.) Their skeletons, which were compact and fused in many parts, not unlike those of birds, were surely well-balanced (powered flight demands a central balance point), despite the relatively large size of their heads. Those heads, by the way, were fenestrated – that is, the bone had openings, or holes, which reduced the weight. They may also have been neumatized, or filled with air sacs.
Pterosaurs, like birds, were probably warm-blooded and laid eggs (though it seems they were soft eggs, unlike birds’ hard ones). We know they were covered with “dino fuzz,” hairlike structures, much like mammal fur, made of keratin – a different brand from that of feathers. They show indications of having developed a respiratory system with air sacs throughout the body, similar to that of today’s birds and equally efficient. Powered flight requires more oxygen than any other land activity, and the tetrapod template had to change to accommodate that. Mammals have a much less effective respiratory system; bats have developed very large lungs to help them with their oxygen needs. But the birds’ and pterosaurs’ way is far more efficient (see the section on respiration in flight, and the separate article elsewhere on this site that goes into avian respiration in some depth).
Many pterosaurs flew over oceans and skimmed the surface for food, as albatrosses and other pelagic species do today. But other pterosaurs, even some of the largest ones, have been found far from ancient shorelines, and were clearly land predators. All pterosaur wings were composed of huge, furry skin membranes stretched via an elongated fourth digit from shoulder to ankle, and laced with blood vessels. They were controlled by powerful muscles in their front limbs as well as pectoral muscles. The membrane was attached to the legs, as well as the shoulder, and the legs were most often connected by a uropategium. The legs seem not to have been directly involved in powering flight, but in stabilizing the wing to keep it from being flipped up at flight speeds like a sail escaping its deck ropes.
These details help give a picture of the differences between bird and reptile flight, and that’s important because it illustrates so clearly that flight apparatus has developed separately a number of times. And it helps us explore the different ways the tetrapod body can achieve the changes necessary for entering a new environment.
How Bird Flight Came About
The earliest actual fossil bird found so far, the Aurornis xui, from 100 million years ago and first described in 2013 (until recently, the Archaeopteryx, about 10 million years younger, was the oldest found), had strong feathers and the skeleton of a dinosaur. How well or even if it flew is still a matter of debate. What we see of its fossil remains shows rounded wings, well-feathered, but nothing like the extraordinary wingspan of the pterosaurs.
The Central Element in the Evolution of Today’s Birds Was the Need for Flight
Flight defines the bird. Even if some avian species have given up flight (ostriches and their kin, for example), the development of flight and the body changes it necessitated are key factors in their lives. All have wings, feathers, bills, light skulls and pneumatized skeletons, no teeth. They have heightened vision and some have other heightened senses; they have quickened responses to stimuli; they have certain vocal “appliances” and a general body shape that, although vertebrate and tetrapod (four-limbed), is unlike that of mammals and reptiles. (Fish, for instance, are vertebrates but not tetrapods.)
So: Why Flight?
Survival values: escape from predators and increased food sources. Social values: display.
Life styles: covering more territory for food and shelter; more dramatically showy – aerial displays are extraordinary and can be seen from miles away. And finally, more easily following changes in seasons.
Costs of Flight
Flight requires extreme energy, more than any other land vertebrate activity. The basic changes also took time: in order to take flight and sustain it, the vertebrate/tetrapod body needed many alterations that put great stresses on the rest of the bird’s functioning.
First of all, energy is expensive: When birds are isolated on islands, for example, where food is abundant and they have few predators, many simply give up flight. Where they need flight, though, the adaptations are ever-refining. As time goes on, bird species continue to adjust their flight tools to fit into their chosen habitats. In a recent example of this, a thirty-year study of a specific Cliff Swallow colony reveals that in order to survive in their chosen habitat (nests under a bridge over a busy road), the birds in that particular population, over a very few generations, have developed shorter wings so that when they drop from the nests to take flight, they can get airborne before their wings collide with on-coming cars. In other words, the shorter-winged individuals survived longer than did their longer-winged compatriots, and reproduced more effectively. Oh the power of natural selection!
What are the necessary elements for bird flight? Wings, light weight; increased oxygenation; fast sense and motor responses (vision and faster neural and muscular response to stimuli); central balance/buoyancy; wings and feathers with airfoil shape, etc.; other feather adaptations for certain kinds of flight; feet capable of grasping food, gripping perches, swimming, grooming. Extremely efficient body systems such as digestion, reproduction, circulation, respiration.
Let’s look at these elements in detail:
Anatomy, Weight, Body Systems and Functions, and Sense & Brain Functions
Weight and Anatomy are Linked
Weight reduction occurs in skeletal, muscle, and body systems, as well as in centralized mass distribution.
Skeleton: The bird’s skeleton is light, not because the bone material is lighter than in mammals: in fact, bird bones are denser – stronger though more brittle. But the bones are thin and have fused, changed shape, some have disappeared. So that while a single bone may be denser in a bird, the overall skeleton may be lighter than in a similar sized mammal.
Weight limits: The bird must be light enough to be buoyant in air; strong enough to withstand the stresses of flight. Today, about 40 pounds is the upper limit for the modern bird body; among pterosaurs, weight was far greater and yet they achieved efficient flapping flight. (Note: hummingbirds weigh about 5 grams.)
Weight reduction is achieved by: Hollow, “strutted,” and fused bones; pneumatized bones; head weight is reduced: the skull is light – there are no teeth and no long nose (beaks weigh very little, though some are extremely powerful).
Bird bones – construction: Birds, as we will explore later on in the section on respiration, have air sacs not only in their body cavity, but in their major bones: skull, neck, humerus, to name the most significant.
Pneumatized bones give birds an advantage over bats, whose bones are not air-filled. The advantage is not, however, simply light weight, because birds and bats of similar size often weigh the same. The edge comes rather in buoyancy. You have only to watch the Red-tailed Hawk in the Nova film Raptor Force picking small bats out of the air in the dim light of after sunset to see this buoyancy and control in action. The hawk is much larger than the bats it is hunting; yet the speed and maneuverability of the hawk gives it a deadly edge.
How can a skeleton with hollow bones weigh about the same as a similarly sized one with solid, marrow-filled bones? The answer, well-explained in Mark Witton’s Pterosaurs (Princeton, 2013) is that most often the pneumatized bones are somewhat larger than the same non-pneumatized bone in a comparable animal. “Bones,” Witton writes, “become pneumatized through ‘invasions’ of pneumatic diverticula, offshoots from the soft-tissue air sacs,” which absorb the bone core as they grow into it. (Remember that bone is constantly remodeling – getting larger and smaller depending on nutrition and usage.)
“The eroded bone is not simply ‘removed’ from the skeleton,” Witton goes on, “but is redeposited elsewhere as part of the same bone. In effect, the bone is ‘inflated’ until it reaches a mechanical limit. In extensively pneumatized animals like birds and pterosaurs, this can lead to bones attaining huge linear dimensions with minimal addition of bony tissue, a consequence of the air sacs . . . pushing skeletal mechanics to its limits. Thus a pneumatized bone can be thought of as an inflated balloon. It weights the same when it’s deflated, but it is stretched to a larger size.”
The walls of these bones become ever thinner as the bone is stretched. “We may think of heavily pneumatized animals as small forms inflated to the dimensions of big ones, making them lightweight compared to non-pneumatized forms of the same proportions.” This explains not only how the bird’s body is buoyant, but also why, when one asks an audience how much a Golden Eagle weighs, the answer is usually in the neighborhood of 20 – 25 pounds, as one might expect of a dog of similar size, instead of the correct 10 – 12 pounds of the eagle.
Look at the skeleton illustration, above, again. Note:
Central body is compact, centered, and balanced – the fused bones form a rigid, lightweight “box” containing the organs, surrounded by bones and muscles, all within the central section of the form. This allows the bird’s body to withstand the pressures of flight. The extremities are controlled by a network of small muscles and tendons connecting wings and legs to the strong central structure and large central muscles.
The pectoral, or shoulder, girdle is an assembly of the breastbone (sternum with keel), clavicles (furcula or wishbone), coracoids, and scapulas. Birds differ from all other animals in having a keeled breastbone, and it forms the basis of their flying capabilities. The major flight muscles are attached to the keel and the sternal plate at one end, and to the humerus on the other. The size and shape of the plate and keel depend very much on the life style of its owner.
Extremities are light in weight, the connections are slim and strong.
Think, for example of the tucked up neck in a flying heron: the legs, long and very thin, stream out behind. The head is folded back between the “shoulders.” Now look at picture of the flight of the crane. On the cranes, both head and legs are extended. This is because the weight of the crane’s head is less than that of the heavily armed heron, whose big beak is used to stab large fish. Cranes eat small water plants and animals and do not need quite such a weighty jaw and beak, though both need the extra length of neck in order to be able to stalk prey in shallow water and catch it efficiently.
Tendons in the wing: The tendons that allow the energy from the muscles to work the wing are vital. Patagial tendons are the most obvious, controlling the patagial muscles that allow the wing to open and close, and to move up and down. The tendons stretch across the top of the wing from the proximal end of the humerus to the wrist, forming a triangle with the humerus and radius/ulna when the wing is opened. This system controls small wing adjustments, working with the alula bone and the small muscles to make flight complex and elegant – aerodynamics and maneuverability are vital to many flight styles. The vinculum, a tendon that threads through the stems of the flight feathers, extends from the distal end of the humerus to one of the digits. It further refines feather movements, adding control.
Tendons in the leg: These control the small lower limb muscles and are as vital in their way as the wing system, allowing refined ground or water locomotion and mobility of the leg and foot for grooming, walking, swimming, grasping perches, killing prey.
Efficient digestive and reproductive system: Intake: birds eat high-energy, high-efficiency food – seeds, fruits, meat. No leaves or other cellulose foods that require long periods of digestion and produce heavy waste.
Elimination: Birds have one orifice for elimination of waste and for reproduction. They have small kidneys and no bladder, so urine, in its form as “mutes” (a white, liquid or semi-liquid substance that dries to “chalk”) is not stored, but evacuated as it forms. Many birds eliminate as a regurgitated pellet those solids that are too large or tough to form the soft, small feces that accompany the liquid “urine.” The pellet is formed in the gizzard and moved, via a kind of peristalsis, back up the esophagus to the mouth.
Reproduction: Fertilization takes place at the cloaca, or vent. Sperm is usually deposited at the opening, because many birds have no penis (some ducks do), and eggs, produced by the female’s single active ovary, are fertilized and descend the canal, getting the hard calcium coating and any distinctive color as they go. (Bird eggs are the hardest among egg laying species.) The reproductive organs are enlarged during breeding season and shrink to a vanishingly small size the rest of the year. The female has only one active ovary.
Extraordinarily efficient respiratory system: Birds have a complicated, interconnected respiratory system of air sacs and relatively static lungs – different enough from ours to require some discussion.
This interconnected system permits a constant, unidirectional flow of air through the lungs, so that the air in the lungs is constantly refreshed and has a high oxygen content. In contrast, in mammals, air flow is bidirectional, moving into and out of the lungs on inhale and exhale. Air entering a mammal’s lungs on an in-breath is mixed with air that has been in the lungs for a while and has less oxygen.
The avian system consists of the trachea, a series of air sacs throughout the central body and the bones, and the lungs. Birds’ lungs are almost inelastic – sacs in which gas exchange takes place. The air sacs are expandable. When birds take a breath, the air goes into the farthest of the air sacs, pushing the air already there forward into the forward sacs and the lungs. Oxygen-carbon dioxide exchange takes place in the lungs and oxygen feeds into the rich blood supply there. The “old” air that was present in the lungs on that inhale is expelled as the new air flows in. On the second inhale, the air in the rear sacs moves forward, and so on, repeating the cycle. In this system, the bird’s bodies are always air-filled and oxygenated. The exhale does not empty the body of air, as does the mammal’s respiratory system. The circulatory system also plays a part in the bird’s air efficiency, since it pumps oxygen rapidly throughout the body. In addition, as the bird flies, the wings speed up the pumping system, further enriching the body with oxygen.
All this of course creates heat, and the bird’s breathing system, feather tracts, featherless legs and feet (except in some owls and arctic species) and complex circulatory system all contribute to cooling the body.
Efficient circulatory system: The avian circulatory system supplies oxygen for flight and aids in thermal regulation. Birds have much larger hearts, relative to body size, than mammals or reptiles do. They have four-chambered hearts, as do mammals (reptiles have three), but their heart rate is much faster. Resting heart rate of songbirds is in the area of 350-450 beats per minute. In bursts of extreme activity, such as flight, it can go to 1,000 bpm. Heart rates go down as body size goes up. Ostriches have a rate of 40 bpm, resting, going up to about 100 bpm in exertion. But ostriches do not not fly. Large hawks and eagles have slower heart rates than, say, small kestrels, but always faster than the flightless birds.
Changes in function: Front limbs changed to wings for locomotion
The problems that follow exchanging “hands” for wings are legion. They include grooming, climbing, communicating via mammal/reptilian-type touch and gesture, grasping/lifting, holding food, escape, and self-defense. Most of these functions must be done by the bird’s beak and feet (many species use their wings in communication, however). And in some instances, feathers and their enhanced vocal apparatus.
These changes are compensated for by: Neck bones increased in number and flexibility (7 cervical vertebrae in humans, 14 in raptors, 22 in swans, plus an axial joint at the base of the skull that also increases flexibility). This allows the beak to function in preening, capturing or securing food, feeding young, self-defense, and communicative sounds. (Owls and many waders clack or clatter their mandibles to communicate, etc.) Some waterbirds have an elongated trachea folded into the keel, allowing for great booming sounds. Of course songbirds have a syrinx, which can produce complex sounds for interspecies and intraspecies communication.) In addition, the bird’s legs and feet accomplish complex tasks: the joints are flexible and allow the toes to groom head and neck, and keep the bird secure on a perch. The feet of most species possess a ratchet mechanism for gripping. Most birds’ legs and feet are centered for balance and walking, and tucked up for flight (except for loons, penguins, and other strong swimmers).
Both beak and feet of all are further adapted to each species’ conditions – raptors use their feet to kill prey, their beaks to tear flesh. Swallows capture insects in their mouths on the wing; dabblers use their serrated beaks to filter food from water, and so on. For all perching birds, the feet grasp and lock for secure perching at night; raptors’ feet lock onto their prey. Ducks and geese have webbed feet for efficient swimming. The refinements are almost as numerous as there are species in existence.
These changes are on-going, as extended life-changes put demands on individuals and species in various habitats. Darwin’s Finches in the Galapagos Islands illustrate multiple changes in beak shape over a relatively short time (possibly thousands of years, rather than millions), allowing an increasing population of finches to avoid competition over food by speciation.
Even more recently, as mentioned above, researchers studying Cliff Swallows in the American Mid-West, hypothesizing that variations in wingspan had a measurable impact on the birds’ survival in some human-dominated habitats, concentrated on a few populations of swifts nesting on bridges and highway overpasses. They found by measuring wing spans and counting mortality rates, that these birds were reducing the number of their flock who were dying on the road. The scientists measured wingspan over a period of about 30 years, and found a significant reduction. The number of birds dying when they dropped from their nests to take off over the road was reduced overall, because the surviving shorter winged birds were less likely to hit oncoming cars. These “new” wing designs are now more prevalent than the “normal” longer wings in these particular populations.
Sense and Neurological Changes:
All avian species have refined the general pattern of adaptation to flight to help them fit into their chosen niches.
Vision in birds is extremely acute, and in nocturnal birds, hearing is also acute. These senses allow for faster reaction time for flight, for hunting, and for evading predators. The centers in the avian brain for vision, hearing, and locomotion are large and complex, much more so than in most mammals.
Reaction time in birds is faster than that of mammals, both muscularly and in recognition of stimuli – examples are flock birds turning as one; peregrines in a dive; all birds landing with precision; and the dance of the airborne predator and airborne prey. Recent research indicates that the areas of the brain dedicated to both vision and reaction time may be increased in both size and in activity under appropriate stimulation.
Avian vision – A recap (please see the article on vision under “Bird Biology” elsewhere on this website): Bird vision takes place partly on the retinal level (that is, the response signal does not have to travel to the brain), which adds speed in reaction to stimuli. In addition, birds have enhanced neural connections between receptors and brain. Diurnal birds have extraordinary color vision, owing to more numerous cones (some birds have about ten million cones; humans have about six million; other mammals have far fewer) and more types of color cones than mammals have (birds have four or five types of cones – depending, we believe, on species; humans and other primates have three; most other mammals, particularly nocturnal species or species that spent a significant amount of their developing time as nocturnal, have only two). Nocturnal birds have increased rods for dim-light vision. We don’t know if they once had rich cone concentrations and lost them, or if they developed entirely through nocturnal lines.
Many birds, including hawks and falcons, but not owls, have two enhanced focal areas on their retinas. (Owls and mammals have only one.) Birds also see the world slowed down, in comparison with us: we see 25 frames per second (as in video) as continuous action. Birds need 60 – 80 frames per second to perceive smooth motion. This is surely necessary for safe high-speed flight.
McGill and Quebec University researchers have recently shown that some populations of certain corvid species make unusual calculations to avoid getting killed by cars when they are feeding by the roadside. They seem not to judge the speed of individual cars, in order to avoid getting hit, but to calculate an average, presumably by observation. Their reactions can be predicted by the speed limit on the road!
On-going studies to watch for: Research on bird senses is intense right now. Keep an eye on the science digests for updates. See New Research & Information on this website, as well.
The Anatomy of Bird Flight
Bird flight is all about getting off the ground and staying in the air. Or aerodynamics.
Lift and drag are the essential elements of aerodynamics. The play between these two defines success of a bird’s flight and style.
Lift = what gets the bird up and keeps it up.
Drag = what reduces lift and thrust and helps the bird land.
Lift is created by air flowing around and over an airfoil. An airplane wing and a bird’s wing are both airfoils, and they look a good bit alike.
Air flowing over the top of the wing must reach the trailing edge of the wing at the same time as the air flowing under the wing. The wing, an airfoil, is curved up on the upper surface, and curved in on the lower. So for the two streams of air to reach the back of the wing simultaneously, the air flowing over the wing moves further than the air flowing under the wing, and it must move faster than the air traveling along the under edge. This difference in air speed between the top and underside of the wing increases pressure above the wing, and this causes lift.
Drag is created by turbulence – or anything that slows the air moving over the wing.
Feathers, like wings themselves, are airfoils. (Illustration to come.) The various structural elements of feathers and their alignment during flight help produce or reduce drag. Reduction of drag allows more successful lift and flight; production of drag allows certain turning maneuvers and landing.
Flight feathers are aerodynamic in shape. The wing feathers are asymmetric – with a larger vane on the outside edge. The faster the flight required, the harder and more aerodynamic the species’ feathers. Falcons have “hard-penned” wings – the feathers are very stiff and rigid, appropriate for their diving flights. Owls have soft wings – the feathers are “feathered” on the leading edges and in general soft and pliable for silent flight.
Feathers move independently. This means that if placed in a certain way, they increase lift of the wing. And if opened and turned, they create drag, slowing the flight.
Further control in flight is aided by a group of feathers at the bend of the “wrist” called the alulae, attached to the alula bone (or digit one), which together act like airplane flaps.
Forms of Flight
The basic flight consists of a down stroke, powered by the pectoral muscles, which give thrust and lift; an upstroke (powered by the supracoidius muscles) – the wing is tucked back in preparation for the next downstroke. (Hummers actually achieve power in both down and up strokes.) Fine movements are controlled by the forelimb muscles and tendons: to raise and lower the leading edge, for example; extend and flex the forewing; control the alula bone for further maneuverability. Look again at the series of photographs showing an eagle in flight at the top of the article.
To get aloft: Most birds tilt the wings and upper body up and and then jump. Some launch from high places (as the California Condor). Some run – large waterbirds run on water until they are aloft.
Birds remain in flight by flapping or adjusting their wings to updrafts and other air movements.
To slow down quickly, birds change the angle of their wings to be higher and higher, increasing drag (to slow their forward movement) and decreasing lift (to help them move downward). Some birds need to slow down for a longer time in order to make a safe landing. Many ducks, geese, and cranes use their outstretched feet as well as their open wings to increase drag, acting as brakes to slow them.
Species can vary in all aspects of flight: for instance, herons fly with the head tucked back; cranes fly with both head and legs extended. This is because those species differ in the food they depend on. Herons fish, and need their heavy sharp beaks to get their food. Cranes forage in shallow waters for smaller edible creatures and for plant material, and their beaks are lighter. Pelicans tuck; ibises extend, and so on.
Wing shape, wing loading, aspect ratio
After lift-off, some air is deflected downward under the bird’s wing. The amount of lift depends on the difference in air pressure on the two sides of the wing. So lift increases as the mass of deflected air increases. The amount of air deflected depends on the shape and size of the wing.
Variation in wing size and shape is expressed in wing loading and aspect ratio.
Wing loading = ratio of total body weight to wing area
Aspect ratio = ratio of wing length to width
A quail has small wings for its weight (high wing-loading) and short, broad wings (low aspect ratio). It takes off fast, flies fast without gliding, and not far because continual flapping is high in energy cost.
Red-tail Hawks have large wings for their weight (low wing-loading) and broad wings (low aspect ratio). They soar easily, flap infrequently. They are not speed demons.
A high aspect ratio indicates a long narrow wing. Maximizes lift, reduces drag. These birds glide at a shallow angle for long distances before having to flap. Albatrosses are classic high aspect ratio flyers.
Bird behaviors both enhance (that is, lead to refinements over time) and predict their flight patterns. So there are few hard and fast mathematical rules in bird flight.