Beaks, Bones and Bird Songs: How the Struggle for Survival Has Shaped Birds and Their Behavior - Roger Lederer (2016)
UPS AND DOWNS
The Animals That Conquered the Air
A bird maintains itself in the air by balancing, when near to the mountains or lofty ocean crags; it does this by means of the curves of the winds which as they strike against these projections, being forced to preserve their first impetus bend their straight course towards the sky with divers revolutions, at the beginning of which the birds come to a stop with their wings open, receiving underneath themselves the continual buffetings of the reflex courses of the winds.
—LEONARDO DA VINCI, “Flight,” The Notebooks of Leonardo da Vinci
Fish gotta swim and birds gotta fly, as the song goes. Without the power of flight, birds probably would not be here today in any significant numbers and certainly not 10,000 species all over the globe. After insects started flying about 350 million years ago, their evolution and success proliferated. Pterosaurs, the only flying reptiles, came into being about 200 million years ago, but how nimble they were is a still-unsettled question. Birds, with their light bodies, strong muscles, sturdy skeletons, and those marvelous feathers, conquered the air about 150 million years ago; bats took another 100 million years to come on the scene.
Watching birds from my kitchen window I marvel at how different they are even though they all have to meet the requirements of flight. The California Towhee scurries across the ground like a mouse, only occasionally hopping into the air; the Black Phoebe launches itself off one perch to land on another and then returns to the original one; the Yellow-rumped Warbler darts among the branches of the crabapple tree; and a crow hops from the top of one redwood and glides to the next. The Nuttall’s Woodpecker flaps and swoops from one tree trunk to another. Swallows ply the open sky searching for insects while swans high above subtly move their wings as they migrate southward. Even with basically similar body forms and complements of feathers, birds have adapted a myriad of flight styles.
Flying is essential to survival for the vast majority of birds. It is not a skill that develops after years of practice, like our learning to play the violin or pole vault—it’s an innate ability that allows birds to find food and nesting sites, escape predators, and avoid weather extremes, in order to exploit habitats and niches that no other animals can. As masters of the air, birds are found everywhere on the planet except the center of Antarctica. For the few flightless birds in existence, the loss of flight came about secondarily.
The flight of birds has always fascinated us. In mythology, Icarus’s father, Daedalus, constructed wings of feathers bound with wax; the wax melted when Icarus flew too close to the sun and fell to the sea. The wings were of bird-like construction with a sequence of overlapping and increasingly long feathers from front to back and a curved upper surface, indicating some early knowledge of aerodynamics. In the 15th century Leonardo da Vinci designed the ornithopter to include a series of pulleys to enhance human power as he rightly deduced that human arms were not strong enough to flap extended wings. A locksmith in Sable, France, made an impressive attempt at flying in the 17th century. He fashioned a set of four wings, two on each side, front and back, and attached them to a rod over his shoulder. With some effort, he was able to flap himself to greater and greater heights, step by step, finally reaching rooftops from which he glided across a river. Over a century ago, a few eccentric inventors created machines that were designed to fly with wings powered by human arms. Since our pectoralis muscles are only 1 percent of our body weight, none of these machines got very far. For an average human adult male to fly, a wingspan of 22 feet would be required. The wings would simply be too long and heavy for him to lift. There have been hundreds of other attempts of human-powered flight and a few have been reasonably successful, like the Daedalus aircraft that an Olympic cyclist piloted and powered for 37 miles in 1987.
A 17th-century painting depicts Icarus falling from the sky after getting too close to the sun.
In Empire of the Air (1881), Louis Pierre Mouillard, a French philosopher and glider inventor, described the flight of birds in an essay. “All my life I shall remember the first flight which I saw of the Gyps fulvus, the great tawny vultures of Africa. I was so impressed that all day long I could think of nothing else; and indeed there was good cause, for it was a practical, perfect demonstration of all my preconceived theories concerning the possibilities of artificial flight in a wind.” Orville and Wilbur Wright observed birds too, especially vultures, moving their wings as they flew, and incorporated mechanisms to warp the wings of the powered Wright Flyer, which first took to the air in 1903. But even after the Wright brothers’ flights, knowledge of how birds flew was minimal. In an amusing but rather embarrassing essay titled Principles of Bird Flight published by the New York Academy of Sciences in 1905, May Cline wrote that birds inhale deeply, making themselves lighter than air, and fly off. Sometimes they inhale too much and explode, hence the pile of feathers you occasionally see in the woods. We know a bit more about aerodynamics today.
On a visit to the Udvar-Hazy Center in Virginia, an annex of the Smithsonian National Air and Space Museum, I saw the space shuttle Discovery, a supersonic Concorde, the bomber Enola Gay that made Hiroshima famous, and all manner of other aircraft. Possessing single and double wings, straight and swept back wings, blunt and pointed noses, sleek and dumpy, they all flew. Without those air machines, we would live in a two-dimensional world because all we can do without them is move horizontally along the ground or water.
To understand a bird’s world we need to know something about flight. I spent six months training for a pilot’s certificate and flew small planes for several years before deciding that I had used up eight of my nine lives. General aviation is not particularly safe, but risk is part of the appeal, like skydiving or paragliding. But I didn’t do it because it was risky; I did it because as an ornithologist teaching students how birds fly, I thought I should have some personal experience. That experience imparted some cold sweat, shaky legs, and nausea. Stalling, diving, making 60-degree turns, flying in turbulent air, maneuvering blind through clouds, and trying to land perpendicular to a stiff wind made me appreciate what birds have to deal with. Watch a flock of geese landing on a rainy, blustery day and you will see what I mean. How do birds do it?
ADAPTATIONS FOR FLYING: SKELETONS, MUSCLES, LIGHTNESS, AND POWER
Every adaptation that an organism possesses came about through evolution. Changes in genes or chromosomes that proffer an organism a bit of an advantage will be assimilated, built upon, and refined, typically in very small increments. The skeleton, muscles, physiology, and behavior of each bird were crafted by natural selection over a couple of hundred million years, resulting in these feathered masters of the air. So what do birds need to become creatures of the air? Besides feathers as airfoils, the requirements are a reduction in body weight and sufficient power and energy to move the wings.
The Skeleton: Let There Be Lightness
Being light makes getting into the air easier and reduces the metabolic cost of flying—and anything that reduces energy expenditure adds to the survival quotient. The largest flying bird that ever existed, Argentavis magnificens, lived about six million years ago in what is now Argentina. With a wingspan of about 21 feet and weighing more than 150 pounds, 16 times as heavy as a Bald Eagle, it flew, but just barely. It had to run downhill or launch itself from an elevated perch and depend on wind currents to keep it aloft, making it primarily a glider. The male Kori Bustard is the heaviest flying bird alive today at nearly 40 pounds but it prefers to walk. When threatened it will run and if the threat continues it will it fly—yet it is just too heavy to fly any distance. Light weight is critical to sustained flight: the heavier the bird, the more muscle power is required to lift it, which requires bigger wings to provide more lift, adding more weight and the demand for even more power.
Even some of the most ungainly appearing birds, such as the Greater Flamingo, have developed adaptations for flight.
To lose weight so birds could take to the air, the beak replaced heavy teeth and many bones fused together or were completely lost. Some thoracic (chest) vertebrae melded, eliminating some individual bones and stiffening the back to support the flight muscles. The lumbar, sacral, and some caudal vertebrae fused with the bones of the pelvis, forming a structure called the synsacrum. This construction stiffens the dorsal skeleton, strengthening the entire body to withstand the stresses of flight and landing. The rest of the caudal vertebrae fused into the pygostyle (Greek for rump pillar)—it is sometimes called the “pope’s nose”—a short bony structure that is covered by muscle and skin and to which the tail feathers are attached.
With a rigid skeleton and wings that are pretty much restricted to locomotion, birds need flexible necks to move their head for essential everyday functions like feeding, eating, and nest construction. If you have ever seen an unresponsive bird that just collided with a window, what looks like a broken neck is simply a very flexible one with 13-25 cervical vertebrae that allow the head of the bird to droop. Mammals, with few exceptions, have only seven bones in their neck, and although some mammals have limited neck movement, others, like the giraffe with 10-inch-long cervical vertebrae, are amazingly flexible.
Some bones of the “hand” (outer part of the wing) have been lost or fused, leaving only three digits instead of the typical five found in most land vertebrates. The “thumb” supports the alula or bastard wing, a small but important structure in flight. A number of bones in the toes and legs are fused or have been lost. A bird actually walks on its toes and what appears to be a backward-facing knee is the metatarsus bone, essentially an elongated ankle. The major flight muscles, those providing power, are attached to a ventral extension of the sternum or breastbone, the carina (Latin for keel). When we carve a Thanksgiving turkey, we usually start by slicing pieces of the side of the breast, making a pile of white meat out of the two major flight muscles, eventually exposing the carina. The sternum is braced by the wishbone or furcula (Latin furca, fork). As the flight muscles compress the thorax during the downstroke, the furcula bends, absorbing some of the stress to the skeleton; on the upstroke, the furcula expands, helping a bit in moving the wings upward.
Have you ever picked up a bird that had died recently? After the body dehydrates, a sparrow-sized bird is almost weightless. Many bird bones are mostly hollow, with struts and airspaces, like the triangular construction of a truss bridge. Physics tells us that a hollow cylinder is harder to bend or break than a solid tube of the same mass and material, and internal struts make the cylinder even stronger. The hollowness of the bone varies among birds: eagles, owls, swans, and cranes have thin-walled arm bones while seabirds, loons, and penguins and other divers have the thick-walled arm bones needed to withstand the stresses of underwater swimming. The hollowness, internal struts, density, and shape of bird bones, as well as the reduction in number, makes for a solid but light “fuselage.”
Dropping weight is also achieved by the reduction of internal organs and seasonal physiological changes. Most female birds (with exceptions like the kiwi) have only a left ovary and oviduct, which shrink noticeably during the non-breeding season. The male’s testes are tiny during the non-breeding season and may enlarge by as much as 300 times when the nesting season commences—at this point the birds are not migrating so they can afford a bit more weight. Except for the Ostrich, birds do not have a bladder. Carrying water only makes birds heavy, so instead of producing watery urine, their nitrogenous waste is in the form of a white paste of insoluble uric acid, which contains only about 5 percent water. When drinking nectar, hummingbirds take on a lot of water and thus weight. To reduce the amount of water they carry, they transpire some of it from their respiratory system—huffing and puffing 250 times per minute—and remove the rest via their very efficient kidneys. A typical hummingbird, in fact, will eliminate twice its weight in water every day.
Muscles and Physiology: The Engines for Flight
Being light is necessary but not sufficient. Although a bird can glide and soar in the wind, the range and duration of flight are limited—and then there’s the problem of getting into the air in the first place. Survival in the sky requires the physical force of muscles and the physiological engine to power them. How else could the amazing Alpine Swift fly for 200 continuous days without landing?
Of the 45 muscles involved in powering and directing the flight of a bird, the two most important are those that comprise the breast. The breastbone of the bird holds the large pectoralis and the much smaller supracoracoideus underneath; together they account for about 17-30 percent of a bird’s total weight. The pectoralis pulls the humerus down and forward for the power stroke. The supracoracoideus, via a tendinous pulley system through the shoulder, pulls the wing up and back in a recovery stroke.
The avian circulatory system, like that of mammals, is driven by a four-chambered heart. The avian heart accounts for about 1 percent of total body weight, twice that of a mammal’s. Smaller birds have relatively larger hearts; a hummingbird’s heart comprises about 2 percent of the bird’s weight compared to a turkey’s heart at less than 0.5 percent. (Data on bird heart weights were gathered by Frank Hartman in 1955 who shot and dissected more than 1340 birds of 291 species, the kind of research that is frowned upon these days.) Bird hearts pump a greater volume of blood per minute, and their heart rates tend to be faster than the hearts of similar-sized mammals. The Blue Jay’s heart beats 165 times per minute, the American Robin’s 550, the Blue-winged Teal’s up to 1000, and some hummingbirds an astounding 1200. Small mammals like the house mouse have comparably high heart rates at 670 beats per minute versus a human’s moderate 60-90.
Bird hearts are more efficient than mammalian hearts because the faster heart rate and the efficiency with which the ventricles fill and empty circulate more blood with each contraction. Avian heart cells are also stronger and more efficient at absorbing oxygen. Blood pressure is also generally higher in birds than mammals with the pigeon’s measuring at 135/105 and the chicken at 180/160. The blood pressure of rats, domestic dogs, and humans are similar at 120/70; the relaxed guinea pig comes in at 80/55. Domestic turkeys, bred for large breast muscles, have very high blood pressure at 235/141, and squabbles among captive birds occasionally lead to heart attacks or aortic ruptures. Once, while banding a Northern Cardinal, I could feel its heart pulsing rapidly and strongly. Before I squeezed the band closed, the bird died, most likely of a heart-related issue. Wild birds have heart attacks, but cardiac problems are much more common in caged pet birds as they tend to be “perch potatoes” and are often fed junk food.
The strong circulatory system necessitates an effective respiratory system. In mammals a muscular diaphragm helps the lungs expand and contract to inhale and exhale air. Birds do not have a diaphragm and their lungs do not move. Extending from the lungs are nine (typically) thin-walled air sacs that, along with the lungs, comprise the respiratory system. The air sacs account for 15 percent of a bird’s body volume, compared to 7 percent for mammalian lungs. Some air sacs penetrate hollow bones like the humerus and fill areas between the skin and muscle. During inspiration the chest expands, and the decreased pressure brings air in through the lungs into the air sacs and then back through the lungs again on expiration. The continual flow minimizes the mixing of new oxygen-rich air and stale carbon dioxide-saturated air as happens in mammalian lungs. Air sacs also serve as a cushion for the viscera upon landing, protection for birds diving from air into the water, and an adjustable buoyancy mechanism for many water birds. Air sacs have also been modified for courtship displays as in the Magnificent Frigatebird, Prairie Chicken, and Sage Grouse. Because birds lack sweat glands, the respiratory system also functions to eliminate heat from the body.
Many birds fly at high altitudes, especially on migration, but their high respiratory rate provides them with all the oxygen they need. Adult humans take 10-12 breaths per minute. An Ostrich’s respiratory rate is five or six breaths per minute, a pigeon’s respiratory rate is 28 breaths per minute, a House Sparrow’s 57, and a European Robin’s 97. The amazing Bar-headed Goose, which migrates over the Himalayas at almost 30,000 feet, has hemoglobin especially adapted to attract oxygen, important for survival at high altitudes where the oxygen levels are 35 percent lower than at sea level. Bar-headed Geese breathe more deeply and at a faster rate when more oxygen is needed; they can also hyperventilate, but without suffering the effects of lightheadedness that humans do.
Male Magnificent Frigatebird displaying to females by expanding its air sacs.
Metabolism and Energy
Exercise physiologists have told us for years that a good workout regimen will lower our basal (resting) metabolism. But if we only sporadically exercise, that workout only increases our resting metabolism. Birds, on the other hand, after a long flight or just a short hop across the field, exhibit a reduction in their resting rate because their metabolism is so finely tuned. Although the basal metabolic rate of small birds is 40-70 percent higher than small mammals, the energy cost of activities above the basic level seems to be no more in birds than in mammals. In fact, flying is more energetically economical than walking, hopping, or running is for large mammals.
As an undergraduate I earned minimum wage by cleaning the bottom of birdcages and separating the poop from the leftover food. The researcher I worked for was trying to determine the energy requirements of molting birds in captivity by measuring their activity, amount of food eaten, and feces produced. This approach gave rough estimates of energy use but more accurate methods were developed later. In a classic (I’d even call it cool) experiment, Vance Tucker put oxygen masks on parakeets and trained them to fly in a wind tunnel. Graphing oxygen consumption against flight speed produced a U-shaped curve, indicating that at low and high speeds energy consumption is higher than at a moderate, supposedly optimum, flight speed. Although this idea has been debated since the 1970s, recent research indicates that Tucker was probably on the right track.
In the past ornithologists would estimate the time and energy budget of a bird in the field by watching and noting its behavior at 10-second intervals. So for each minute of observation one would have six data points describing the bird’s actions—feeding, flying, perching, sleeping, chasing, or whatever. Five hours in the field would yield 1800 notations. The work was tedious and not particularly stimulating, but by combining those data with lab studies on how much energy birds use, we can derive an understanding of how much energy birds need to survive in the wild on a daily basis. With today’s methodology and technology, including fitting free-living birds with electronic gear that transmits heart and respiratory rates, ornithologists have accumulated substantial data about the energy required for flight.
The energy needed depends on the size of the bird, wing shape, the mode of flight, and environmental conditions. To estimate energy needed for a particular activity, researchers first determine the basal metabolic rate (BMR), which is the energy used while resting in a thermoneutral environment (no extra energy used to cool or warm the bird). The flight energy requirements of hummingbirds, for example, are 12-15 times their basal metabolic rate, so they forage for nectar, which is high in sugar. John Videler’s book, Avian Flight, reports energy used during flight as a multiple of BMR. Some examples: Barn Swallow 5.1, European Starling 10.3, Rock Dove 18.0, Red-footed Booby 2.7, and the Wandering Albatross—the most efficient flier—at 1.4.
FEATHER AND WING SPECIALIZATION
Several avian adaptations make flight possible, but feathers are truly indispensible. Feathers are made of the protein keratin (as are beaks, scales, and claws), rendering them light and soft but also strong and flexible. A unique feature of birds, feathers could not have come about de novo into their present form; they arose slowly from some simple basic structures into the array of forms we have today. Archeopteryx, long considered an intermediate between reptiles and modern birds, had wings with feathers very much like the flight feathers of today’s birds. Debates arose about how these wings were used. Were they used for powered flight, gliding, or something else? Few scientists recognized that the feathers of Archeopteryx were pretty advanced and that natural selection apparently molded them many millions of years earlier from more primitive structures. Like most ornithologists, for many years I accepted the hypothesis that feathers evolved as an extending, flattening, and thinning of reptilian scales. This made sense since scales and feathers are made of similar material and birds evolved from reptiles. As dinosaurs, then mammals and birds, evolved, so did homeothermy (warm-bloodedness) and it made sense that elongated scales came along to provide insulation.
But Richard Prum, an ornithologist at Yale University, in a flash of insight, wondered about feather origins and eventually gathered enough evidence, with help from colleagues and students as far away as China, to determine that feathers were actually novel structures that evolved independently. The first clue to this revelation was the realization that scales develop as flat structures and feathers develop as curled tubes that unfold. The first feathers were little fuzz-like structures in shades of black and brown. To make a long story short, it appears that feathers evolved for visual communication, display—and that’s what kept them moving on the evolutionary path toward longer and more refined structures that along the way became useful for insulation (as down feathers), gliding, and eventually flying.
Because of their ability to endow birds with flight, feathers have been imbued with magical and mystical powers over the ages, but birds are still subject to the laws of physics. The four forces of flight are lift, gravity, thrust, and drag. The wings and tail provide lift to counteract the force of gravity. When birds flap their wings in a downstroke, the wingtips move forward and down, producing forward thrust, sort of like humans swimming the crawl stroke. Thrust from the flapping wings propels the bird through the sky, counteracting gravity, producing lift, and overcoming drag due to the friction of air moving over the bird. Then the wings reverse in a recovery stroke and move upward and back while flexed inward to minimize drag.
When a bird’s body accelerates forward, lift is proportional to thrust—the faster the bird flies, the more lift is generated. But if the bird angles upward to gain altitude, as it does on takeoff, for instance, the angle of the wing increases relative to the oncoming air. This is the “angle of attack.” If the angle of attack is too great, the wings lose lift, causing a stall. The alula, the few feathers attached to the thumb, can rise and form a slot over the wing, smoothing the air and allow a higher angle of attack without a stall.
Lift and thrust move a bird through the air, but to steer, brake, and land birds have specialized contour feathers. Each contour feather has a shaft with parallel barbs emanating on both sides to form a flexible vane. The portion of shaft extending below the vane is the calamus or quill. You are probably familiar with how these feathers feel: you can run your index finger and thumb down to separate the barbs and make the feather ragged; just as quickly, like Velcro, if you run your fingers upward along the vanes the feather barbs can be neatly reattached.
Forces of flight.
Different contour feathers perform different flight functions. Producing thrust, the 9-11 primary feathers attached to the hand have different-sized vanes on each side, making them asymmetrical; they can be individually rotated to reduce drag and turbulence by forming horizontal slots between them, aided by notches near the ends of the feathers. The secondary feathers of the arm, 9-11 in songbirds and up to 40 in albatrosses, are symmetrical. They move in unison because they are connected across their bases by a ligament that allows them to be moved simultaneously to provide lift or help in landing. Three to five flight feathers are attached to the thumb, forming the alula, which helps to reduce turbulence over the wing when the alula is lifted to form a slot.
Particularly important are the primary and secondary wing coverts (think “cover”)—the feathers on the leading edge of the wing that smooth airflow by forming a tapered surface to cover gaps between the bases of flight feathers. The feathers of the shoulder, the tertials and scapulars, fill in aerodynamic gaps and help with lift. There are covert feathers over much of the body including the tail, filling in gaps to form an aerodynamic shape.
Bird wing indicating contour feather types.
Attached to the muscles supported by the bony pygostyle are the tail feathers, which spread, fold, and move (up, down, and laterally) to enable lift, turning, and braking. Cats and other predators that attack birds from behind to avoid detection will often end up with a mouthful of feathers as birds occasionally drop their tail feathers in a “fright molt” to distract a predator. But trading tail feathers for escape to safety means that the bird’s life will be more difficult until the feathers grow back in six weeks or so. The bird has to fly faster to compensate for the loss of lift and the aid of the tail in turning and braking, and the wings must work harder.
The color of some feathers plays a surprisingly important role in flight. The pigment melanin, which produces brown and black colors, increases the resistance of feathers against abrasion from the turbulent air around the wingtips, airborne particles, the wear caused by feathers rubbing against each other, and degradation by feather bacteria. This is why many large birds (Snow Geese, White Pelicans, the European White Stork, and some gulls and terns) have black or dark wingtips, or totally black wings (like frigatebirds and murres).
Configuration of Wings
When leading bird hikes, I advise participants to focus not on color for identification, but on the bird’s shape. In 1934, Roger Tory Peterson, a well-known ornithologist and artist, published a field guide to birds and included on the inside covers (a format that would became the model for subsequent guides) silhouettes of birds perched and flying because shapes are so important in identification. A significant part of a bird’s silhouette, especially in flight, is its wings. Many variations in wings have arisen as different habitats have molded birds for different niches, each requiring its own mode of flight. Short and wide, long and narrow, and all manner of intermediate wing shapes have evolved, including those for underwater “flight.” Like the beak, the wings are elegantly suited to carry on the lifestyle of a particular bird. But, as in airplanes, the laws of physics circumscribe the limits of wing variation.
Two measures of a bird’s wing, the length and the width, tell us a lot about the bird’s flying style. The wing aspect ratio is the ratio of the length of the wing to its width. Wing loading is the relationship of weight to wing area; the less weight a wing has to support, the less energy a bird has to use. Soaring and gliding birds have bigger wings and thus have smaller wing loadings than songbirds, so they need less power to generate lift and can soar and glide without using much energy. Birds like chickadees and bullfinches cannot soar and can glide only for short distances because their wings are too small to produce sufficient lift without considerable power. Most bird wings can be categorized as one of four types: high aspect ratio wings; high lift, low aspect ratio wings; high speed wings; or elliptical wings.
Wing aspect ratios.
HIGH ASPECT RATIO WINGS are much longer than wide and are used for slower flight as seen in storks, eagles, frigatebirds, pelicans, albatrosses, and seabirds that frequently hover or soar. These wings have deeply notched primary feathers with narrow, pointed wingtips to minimize turbulence and enable long periods of soaring, like the weeklong flights of frigatebirds. Like a tightrope walker with a balancing pole, long wings provide stability and are less susceptible to jostling by air currents. On the other hand, they cannot be used for quick maneuvers. Big birds with high aspect ratio wings usually need to take off from an elevated perch or into the wind. A stiff wind is helpful for landing, which could otherwise be difficult. Military servicemen stationed on Pacific islands during World War II gave the nickname “gooney bird” to the albatross after watching these birds make ungainly and often clumsy takeoffs and landings.
HIGH LIFT, LOW ASPECT RATIO WINGS are broad (wider but shorter) wings found on many raptors like hawks, eagles, vultures, and the Osprey. The birds can soar, glide, and maneuver well, but without much speed. High lift wings don’t have the stability of high aspect ratio wings and they produce more drag, but they provide enough power to soar on thermal updrafts and lift heavy prey. Slow flight is important to predatory birds and scavengers because it allows the birds to scan for potential food sources. The feathers of these wings are curved in cross section to provide lift and the slots formed at the tips of the wings by the primary feathers help to reduce wing tip turbulence (vortices).
HIGH SPEED WINGS are long and pointed and common in ducks, falcons, auks, as well as birds that feed aerially like swifts, undergo long migrations such as shorebirds, or “fly” underwater like puffins and murres. The wings taper to a point to minimize drag and energy consumption but require fast wingbeats to produce lift. Slow flight is difficult.
ELLIPTICAL WINGS are short and rounded to allow for quick maneuvering in tight spaces like forest and scrub habitats. Crows and many other songbirds have elliptical wings for quick changes of direction and minimal drag. Their wingbeat is typically rapid and the primary feathers are strongly slotted to prevent stalling during quick turns and frequent landings and takeoffs. The Cooper’s Hawk pursues its mostly avian prey with a quick burst of speed and agile avoidance of tree branches and shrubbery. The tradeoff for agility is high energy use.
Tundra Swans have a wingspan of nearly six feet, which provides the birds with considerable lift. They slide smoothly through the air using almost imperceptible movements of their primary feathers. Canada Geese flap a little harder and faster as they have smaller wings, but again mostly with their primary feathers. Ducks, especially fast-flying teal with smaller wings, need to flap rapidly and use most of their wing surface for lift and thrust. The small wing area of many songbirds provides minimal lift, so they have to flap even faster and harder to stay aloft, at least 11 mph, perhaps all the way from the tundra.
As a bird flies, it adjusts its speed and altitude by flapping faster or slower. To change direction the bird adjusts the lift of its wings. If the left wing is pulled in toward the body, lift will decrease on the left side. The fully extended right wing is producing more lift so the bird tips and turns to the left. However, this causes a “skid” in the air—think about making a too fast left turn in your car. To minimize skidding, the tail twists in the direction of the turn, like an airplane rudder. To lose altitude, both wings are pulled inward. All this is difficult to discern unless you watch a slow flying bird to see the constant movements of its wings and tail.
Equipped with long and wide wings, Tundra Swans merely need to move their wingtips to almost effortlessly ply the skies.
Birds have sophisticated ways to deal with the vagaries of air currents. On a blustery day, flying can be a real challenge. In Wyoming (where a wind gauge consists of a bowling ball on a chain, 45 degrees indicating a light breeze) I’ve watched Tree Swallows trying to catch insects in a gusty wind. They dipped, bobbed, dove, and turned so quickly that it was impossible to determine exactly what adjustments the birds were making. A wind tunnel experiment on Barn Swallows provides some explanation. The air in the tunnel was kept constant (no turbulence) while the researchers measured the birds’ flight responses between 8 and 33 mph. Wingbeat measurements showed that their wings beat faster at high and low speeds than at intermediate speeds. Wingbeat amplitude (the distance the wings moved above and below the body) increased with flight speed. Wingspan, the width of the tail spread, and the tilt of the body all decreased as the birds flew faster. What the birds were doing was increasing lift at low speeds and decreasing drag at high speeds. It is hard to imagine how accurately and quickly those Tree Swallows were able to adjust in winds that continuously changed both speed and direction.
Hummingbirds do something a bit different. While most birds get thrust from the downstroke and then bend their wrists back to pull the wing up in a recovery stroke, hummingbirds rotate their wrist after the forward stroke to power the backstroke which also produces lift. Imitate a flying goose or gull by flapping your arms forward and down and then up and back, sort of like swimming. To imitate a hummingbird, hold your upper arms against your side and flap your lower arms the same way. Since the hand and primary feathers comprise 80 percent of a hummingbird’s wing, it is not surprising that they do most of the work. The pectoralis and supracoracoideus muscles of a hummingbird are about equal in size, compared to the 80:20 percent ratio in other birds, and the weight of these two muscles comprises 30 percent of the hummingbird’s total weight, almost twice that of most birds.
TAKING OFF, LANDING, SOARING, AND GLIDING
Before a bird can flap, soar, or glide, it has to get itself into the air. What steps got birds into the air? Paleontologist Xing Xu and his colleagues in Beijing reported that Microraptor, a four-winged dinosaur of the Cretaceous period about 125 million years ago, could glide, representing an intermediate step to the flapping flight stage. It had long flight feathers both on the legs and wings, a fan-shaped tail, and light and dark bands of proto-feathers providing coloration, including iridescent black.
For years two competing theories explained how birds became airborne. The arboreal theory proposed that the birds climbed into trees and glided down. Some evidence supports this theory. Models of Microraptor were put in a wind tunnel at MIT and the lift and drag forces measured. With all four limbs extended horizontally, the model was an excellent glider. The opposing cursorial theory says that birds ran along the ground and eventually generated enough speed to produce lift for takeoff; evidence for this idea is weak. In 2003, a third idea came into play: birds used their wings to climb over rocks or scale walls, flapping and running over steeper and steeper inclines until they were airborne. Using Chukar Partridges, researchers looked at the development of the ability of newly hatched birds to scale an incline, from crawling on all fours to using symmetric wingbeats to fly by 20 days after hatching. They speculate that the development of flight in these partridges mirrors the evolutionary steps to the power of flight, but the debate continues.
Microraptor: speculative illustration of the animal in flight.
Some birds jump off cliffs or tree branches, run into the wind, or run across the top of the water, but a number of birds just launch themselves into the air from wherever they are. Go to the nearest pond or lake and look for a Mallard, the most common duck in the Northern Hemisphere. It has a rather stout body and average wing dimensions, but watch it take off at a 60-degree angle! How can that be? First, it spreads its wings fully and flaps downward, the wrist (between the primary and secondary feathers) hitting the water to lift itself off the surface to become airborne. If a bird or airplane is aimed too steeply upward, the smooth flow of air over the wing is disturbed, lift is diminished, and the bird (or airplane) stalls and falls downward. The Mallard is able to take off steeply but avoid a stall by extending its alula, the three to five feathers attached to the thumb at the wrist. The alula lifts up like slats on an airplane wing and smoothes the air passing over the wing, reducing turbulence until the bird levels off.
When you’ve got fancy plumage, you have to be a bit more careful. With its green tail symbolizing lush plant growth, the Resplendent Quetzal of Central America helped inspire the Mesoamerican legend of Quetzalcoatl, the feathered serpent and god of vegetation. The quetzal avoids injuring its iridescent tail—which is nearly twice as long as its body—by launching itself backward to clear its perch and get airborne.
Landing requires even more skill and agility than taking off. Having landed a plane several hundred times, I can attest that putting the plane in the right place at the right speed is the hard part. To land, airplanes and birds have to approach the ground at the correct speed—too fast and they hit the ground too hard (or run out of runway), too slow and they stall and crash. Just prior to landing, a bird moves its body closer to the vertical, extends its alula, and spreads its tail feathers as a brake while beating its wings against the direction of flight. Waterfowl, cranes, herons, and others use their forwardly outstretched feet as well to increase drag. Constant readjustment of wing flapping speed and position usually put the bird down gently. Studies of pigeons landing indicate that they land with more force on a familiar perch than they do on an unfamiliar one. In other words, they are more cautious when landing in a new place. They also need accuracy. A bird has to land in the right spot; a sparrow can’t land in a puddle and a loon can’t land on the ground.
Landing in a gusty wind is another story. Airplanes have to land in a particular direction—down the runway—no matter what direction the wind might be. Landing in a stiff crosswind is so difficult airplanes have maximum crosswind ratings. Birds have the advantage of almost always being able to land straight into the wind, but they still have to compensate for changes in air currents. Each bird has to spread and twist its wings, tail, and individual flight feathers millisecond by millisecond to put down gradually.
Woodpeckers, nuthatches, creepers, and other birds that feed on tree trunks approach landing differently. They fly close to a tree, spread their wings to slow down, swerve upward along the trunk to dissipate their speed, and grab on when their momentum has reached nearly zero. Woodpeckers have two toes in front and two in back plus a very stiff tail, so they can hang on tightly and creep upward easily. Taking off, they just have to let go, push themselves away from the tree, and spread their wings.
Soaring and Gliding
Paper kites, the first successful implementation of the principles of flight, have been around for at least three millennia. In the years 550-559, the emperor of China’s Northern Qi province studied bird flight but had an odd way of experimenting. He strapped bird-shaped kite-wings to the arms of prisoners and pushed the poor guys off cliffs or towers. Many years later and more humanely, men were lifted off the ground by kites attached to ropes. Kites fly tethered by a string, deflect the wind downward, and soar upward. Birds do the same. What keeps them the air? A bird’s wing is curved in cross section with the top slightly convex. When the bird is airborne, molecules of air hit the forward edge of the wing and separate, some going above and some below. To meet at the back of the wing, the molecules of air above move farther and thus faster than the ones below and as a result the air molecules above the wing are spread farther apart. This difference makes the air above less dense than that below, so air pressure under the wing is greater and the wing is lifted. The faster the wing moves, the greater the lift. The tail helps in the same way.
A Bald Eagle landing at its nest.
Moving in three dimensions requires control in all three. Think about riding in a car when it brakes quickly. The front of the vehicle goes down and the back up, demonstrating pitch. Now the car quickly turns a corner and the car tips in the direction of the turn; this is rolling, the tipping of the body to one side with the axis of the roll through the body from head to tail. A car skidding on snow or ice on the road demonstrates yaw, the horizontal movement around the center of the body, like a weather vane moving in the wind. Pitch, roll, and yaw are minor in a car, but in the air are significant movements that a bird must control in order to be a successful aviator.
Once a bird gets airborne, it can glide, ultimately to the ground, or it can soar, staying aloft using air currents. Birds may use thermals, updrafts of warm air that can form when pockets of adjacent air are different temperatures—over mountains, next to bodies of water, or above paved roads. The birds are lifted by the warm air and move in the desired direction by trading altitude for distance. Birds also use obstruction currents to gain altitude. When wind hits a mountain, cliff, or building, soaring birds use the rising air along the edge of the obstruction. There’s also just plain wind. Birds can hover almost motionless in a stiff wind, adjusting their wings, and gain considerable altitude that they can trade for distance. Watch vultures and large hawks ply their skills the next time you see them.
Perhaps the most ingenious use of minimal power for flight is dynamic soaring, employed by some seabirds, especially albatrosses. When the wind is about 10 mph over the ocean, an albatross heads into the wind, gains altitude, and then turns in the direction it needs to go, trading height for distance. Gottfried Sachs of the Technical University of Munich and colleagues using miniaturized GPS transmitters tracked free-flying albatrosses and found that the birds could fly for thousands of miles using dynamic soaring with little energy expenditure. The researchers speculate that robotic airplanes could be invented that could roam the ocean with little fuel.
Soaring Andean Condor showing wing slots between primary feathers.
Soaring is such a successful technique because slots at or near the end of the wings created by the horizontal and vertical separation of the primary feathers act like winglets on modern airplanes and smooth air over the wingtips. But soaring and gliding are limited to those birds with low wing loadings.
FLOCKS AND FLYING IN FORMATION
Augury, from late Middle English, means foretelling a future event by studying bird flocks. During the Roman Empire, a member of the College of Augurs was always consulted before a battle, election, or starting a major building project to assure that the gods were in favor of the plan. A flock of birds was usually taken as a portent, good or bad.
We have all seen masses of shorebirds, blackbirds, or starlings swirling and swarming every which way in seeming unison. Look at a video of the winter roosts of millions of blackbirds in the Great Dismal Swamp in Virginia. How do they control their movements? Their flight seems almost choreographed. High-speed photography and computer simulations of bird flocks tell us that a bird tracks the movement of its neighbors and adjusts its flight and spacing to stay in sync. A lot of jockeying and readjusting goes on but it happens too quickly for us to notice (recall flicker fusion). George Young and colleagues at Princeton University performed computational studies on the movements in starling flocks and concluded that the birds coordinate their movements with their nearest seven neighbors and that the shape, not size, of the flock matters. According to the researchers, the optimum flock shape is that of a thick pancake: too thin and the birds have a limited view of the flock’s movements, too thick and the birds can’t keep track of the additional neighbors.
Why do birds flock like this in the first place? One reason is safety in numbers: more eyes to spot predators and perhaps confuse or even mob them. More eyes also mean a greater likelihood of locating food or suitable habitat on a migratory route. Additionally, many young songbirds need the social stimuli of the flock to learn the appropriate behavior of their species. Sometimes surviving requires depending on others.
A flock is organized in a hierarchy with higher-ranked birds toward the front. A study involving a 15-bird pigeon flock fitted with geotransmitters provided the exact location of each member of the flock five times per second. It turns out that the leading bird makes the navigational decisions for the flock, but when the leading bird drops back, another high-ranking pigeon takes its place and leads the flock. So the flock has a leader but the leadership position changes often. If you watch a flock of geese you will notice the same phenomenon. The lead position changes often probably because the leader receives no aerodynamic benefit and needs a rest. I remember seeing a television show when I was a kid, when the early nature programs were from the perspective of hunters, showing Canada Geese flying in formation. The narrator said, with all seriousness, that if the leader of the flock was shot, the entire flock would become disoriented and lost. Nope.
Drafting is a common energy-saving technique in bicycle road racing in which a fast biker produces a turbulent wake, thereby producing a low-pressure center that the following biker can ride into and that will help propel him or her forward. Likewise, a soaring bird producing lift creates an upwash of air behind the tip of its wings. These tip vortices, similar to small tornadoes, are helpful to birds flying in a V formation. Theoretical calculations indicate that 25 birds flying in a V, with each bird except the lead flying over the upwash of the bird in front, have a 70 percent increase in range as compared to a lone bird. Henri Weimerskirch and colleagues at the French National Center for Scientific Research implanted heart rate monitors under the feathers of eight pelicans trained to fly in a V over the Senegal River. By measuring the heart rates, they could determine that optimal spacing saves energy and improves communication. A 2014 article in Scientific American reports a study of captive-bred Bald Ibises flying with attached GPS loggers that counted wingbeats and found that the birds not only place themselves in the best position to take advantage of the V formation but also flap in coordination with the wingbeat of the bird in front. Seems like they have it down to a science.
V flight of Canada Geese.
Brown Pelicans, cormorants, and other waterfowl often fly in a line just above the surface of the water. One reason for this behavior is that wind speeds diminish close to the water (or ground) because of friction. Flying close to the surface of the water, the birds also take advantage of “ground effect.” When a bird or airplane is less than a wing’s length above the surface of the ground or water, tip vortices cannot form and the air is compressed between the wing and the water (or ground), increasing lift. All these techniques save energy, which means less time is needed for foraging, and exposure to the elements and predators is decreased. Smaller birds like pigeons and starlings don’t take advantage of V formations or ground effect because their wings are too small to produce a significant upwash of air or benefit from increased pressure from below the wings.
FLYING UNDERWATER AND FLIGHTLESSNESS
Variations on the theme of flight continue with birds that both swim and fly and birds that have completely traded flying in the air for flying underwater. Each of these lifestyles matches the demands of the birds’ environment.
Most birds that are good swimmers are not particularly good fliers because they traded some flight adaptations for swimming ones. However, some birds are both good swimmers and competent fliers such as the ocean-living Pelagic Cormorant and Thick-billed Murre. A study found that the cormorants used a lot more energy to swim than similarly sized penguins did. The murres proved to be more efficient swimmers than the cormorants but still used 30 percent more energy than penguins. To be any more efficient at swimming the murres would have to reduce their wing size or put on more muscle, either one of which would make it impossible for them to fly. The same principle applies to seabirds like petrels, guillemots, and shearwaters: the better the swimmer the worse the flier. Diving petrels are an exception as they can fly quickly over the water and dive in, fold their wings slightly, and swim rapidly under the sea surface for short distances.
Terns, gannets, and some pelicans dive after their prey and are reasonably agile underwater but their hollow bones and buoyant bodies let them penetrate only a short distance under the surface so they must nose-dive into the water, sometimes from considerable heights. I once visited a Northern Gannet colony at Cape St. Mary’s, Newfoundland, where 6000 pairs of these birds breed. (“Cape St. Mary’s pays for it all” was a saying of fishermen who could catch a boatload of fish off this productive coast in years past). Gannets dive from up to 100 feet high, folding their wings back and piercing the water like an arrow. Their eyes are placed forward for wide binocular vision, they have no external nostrils, and their upper chest cavity has air sacs to cushion impact with the water. They plunge into the water and swim after fish using their webbed feet for propulsion and partially extended wings for steering. After a few moments they bob to the surface.
Thick-billed Murres are very efficient swimmers and capable fliers.
Most birds that swim in fresh water propel themselves with their feet. Loons are expert swimmers and divers and have been recorded at depths of 600 feet. Their bones are solid, and their legs are laterally flattened, far back on their body, and terminate in webbed feet. Agile in the water but awkward on land, the loon got its name from the Norwegian word “lom” meaning a clumsy or dull-witted person.
Grebes have lobed toes, not webbed feet, but they are still fine swimmers and can dive down to 130 feet. Johannsen and Norberg of Harvard University filmed swimming Great Crested Grebes and noted that their asymmetric toe lobes and separation between them increased lift as each toe acts as a separate hydrofoil. Like loons, grebes have legs that are placed far back on the body, but unlike loons, grebes can run a short distance on land and even on water. Western and Clark’s Grebes are well known for their “rushing” display during courtship. A pair will lunge out of the water and run together across the water’s surface at 15-20 steps per second for up to 65 feet, without using their wings. I watched this display for several summers on Eagle Lake in northeastern California where I conducted a census of their population, one of the largest in North America. It is always an amazing sight to see birds running upright across the water’s surface.
Dippers are unusual in being highly aquatic songbirds. They dive into rushing streams and swim with their strong wings as they search for aquatic insects. A flap covers their nostrils, their plumage is unusually thick, and their blood holds a high concentration of oxygen. Their feet play little role in locomotion, but the long toes and claws help the bird hold onto rocks. An enlarged preen gland ensures waterproof feathers and their nictitating membrane protects the eyes and assists in underwater vision. Once called the water ouzel, it was renamed because of its habit of dipping and bobbing along stream edges. I have seen dippers nesting on ledges behind waterfalls, which required the parents to fly repeatedly through the waterfall to forage downstream and fly back again through the falling water to the nest.
The Atitlan and Junin Grebes of Peru and Bolivia and the flightless cormorant of the Galapagos traded their ability to fly to become more aquatic. Penguins, which spend about three-quarters of their lives at sea, also gave up flying in the air, but not in the water. They move through the sea with their flat, solid, wings and strong flippers that twist at a 20-degree angle, producing thrust on both the up and down strokes, a bit like hummingbirds. Penguins swim with their heads against their shoulders and feet held against the tail to reduce drag and help in steering. Considered by many to be the most hydrodynamic shape in the animal kingdom, the penguin body has inspired the shape of submarines, torpedoes, and underwater vehicles. According to The Penguins, the birds generally travel at about 5 mph but can exceed 15 mph.
Only about 0.5 percent of birds are unable to take to the skies, but for some it has worked well: besides penguins, the birds include kiwis, Ostriches, cassowaries, rheas, Emus, tinamous, five waterfowl, Kakapo parrots, one cormorant species, two grebe species, and 21 species of rails, coots, moorhens, and crakes. Flightlessness did not evolve as a separate branch of the bird world; flightless birds from various families regressed from their flying ancestors. Regression is not unusual in evolution; consider hairless humans, blind cave fish, and legless snakes. We know that natural selection chooses the more advantageous adaptations over the less useful ones, so why did flightlessness confer a survival advantage over flight in the case of these birds?
The flightless Titicaca Grebe, an endangered species.
Several flightless birds once inhabited Hawaii, New Zealand, and other oceanic islands. Before the arrival of the Polynesians in 900 AD there were perhaps 30 species of flightless birds on New Zealand, 25 percent of all the bird species there. New Caledonia, Madagascar, Jamaica, and many other islands had flightless birds, the best known being the Dodo of Mauritius. These birds could not have come from one branch of the avian tree, so flightlessness clearly evolved in different groups of birds in different places.
Flight allows escape from predation by land animals, so in the absence of predation the selection pressure to maintain flight is absent. But since many island birds continued to fly, the absence of predators can’t be the whole story. It may be that as more birds arrived on the islands and as competition for food increased, some birds reduced their pectoral muscle mass, reducing their basal metabolic rate. As smaller, flightless birds, they need less energy and thus reduce competition for food.
Flight does not belong to birds alone but this characteristic transcends all other features of birds. The grace, elegance, and apparent effortlessness of a flying bird is perhaps the most moving sight in nature. Memorable documentaries like Winged Migration, Earthflight, Fly Away Home, and In-Flight Movie give us views of birds we could never experience otherwise and provide a glimpse of what it might be like to be airborne and untethered. Watch the next birds you see and your appreciation of their freedom from the bounds of the earth will be heightened by this spectacle of evolutionary design that enhanced their lifestyles and ensured their survival.