Birds
Dinosaurs aren’t extinct. There are about 10,000 species alive today in the form of birds. ~ English vertebrate paleontologist Robert Benson
Only 4 groups of terrestrial vertebrates evolved: amphibians, reptiles, mammals, and birds. The outlier of these is the one with wings and feathers, as well as a brain unlike any other. Birds are a highly distinctive animal.
Like mammals, birds are endothermic. Bats can fly and humans are bipedal, but there is little commonality otherwise between mammals and the class called Aves.
Many traits associated with flight evolved before the origin of Aves. ~ English evolutionary biologist Mark Puttick et al
Birds originated from maniraptoran theropod dinosaurs at least 160 MYA. Some basal avian traits appear a case of convergent evolution with earlier reptilian forms, as well as the endothermy found in mammals.
Though birds evolved at a quicker clip than other theropods, the avian body plan gradually came together, with no single leap to point to for the birth of birds.
Growth Evolution
Birds became itty-bitty by truncating the juvenile growth spurt that gave dinosaurs their girth. The shrinking process started 210 MYA and proceeded apace for 50 million years. This appears a reversion back to the earliest dinosaurs, who were quite small.
Miniaturization afforded flight and the novel ecological opportunities that went with it. Besides the aerodynamic advantage of carrying less weight, smaller animals can flap their wings faster than large ones.
After growth slowed to keep size down early in their evolution, avian growth sped up again, to rates even faster than the extinct dinosaurs they descended from. This let a bird mature quickly. By the end of the Cretaceous, a bird the size of a sparrow could grow up in a week’s time.
○○○
Numerous lineages of paravians were experimenting with different modes of flight through the Late Jurassic and Early Cretaceous. ~ Michael Benton et al
While wings ultimately developed to afford being entirely airborne, even early gliders gained aerodynamic benefits. Most flying animals, whether insect or bird, are close to the ground or water when taking off. The surface acts as an aerodynamic mirror, increasing pressure underneath the wing, interrupting downwash, and suppressing wing tip vortices’ turbulence. This aerodynamic interaction lowers the energy required for lift by 29%. Early evolved birds flapping their wings would have been able to run and jump swifter even if they never left the ground.
The biodiversity of these small, bird-like dinosaurs was incredible. ~ Belgian paleontologist Pascal Godefroit
Yi qi
Yi qi lived in northeast China 160 MYA. It had feathers, though these were too insubstantial for flight.
Yi qi managed to glide through air with bat-like wings; one of many evolutionary experiments by which small dinosaurs were airborne.
Aurornis
The pheasant-sized Aurornis arose 160–150 MYA. A half meter from beak to tail-tip, it had a mouthful of tiny sharp teeth and a long tail, with legs adorned in plumage.
Aurornis could not fly, but it had long forelimbs that may have helped it glide through the Jurassic forests. Its hips mark it as related to modern birds.
Archaeopteryx
The raven-sized Archaeopteryx lived 150 MYA, during the Late Jurassic. It was a bird-like dinosaur, with 2 feathered wings.
Archaeopteryx had jaws lined with small sharp teeth, 3 fingers ending in curved claws, and a long bony tail; traits shared with its dinosaur cousins. While these features would be forsaken by its avian descendants, Archaeopteryx had some bird-like traits.
Birds need a sophisticated sensory system to quickly process the flood of visual information that comes during flight, integrated to a complex mapping system. Archaeopteryx had a broad cerebellum, indicative of heavily relying on vision.
Archaeopteryx had a larger relative braincase than dinosaurs. The structure of its inner ear more closely resembled modern birds than reptiles. Archaeopteryx appears to have keen senses of hearing, balance, spatial perception, and coordination; the processing perquisites of flight. Whether and how well Archaeopteryx actually flew remains speculative. Its long tail precluded taking off from the ground.
The fingers on Archaeopteryx‘s wings would have interrupted smooth airflow but they would have been essential in climbing up trees.
Archaeopteryx may have glided decently. Its feathered wings suggest modest flight capability. Albeit distinctive, Archaeopteryx had pennaceous feathers like modern birds. Though the feather structure was weak by modern standards, Archaeopteryx had matt black plumage.
In modern birds, black melanin pigment substantially increases the strength and durability of feathers. ~ American paleontologist Ryan Carney
Archaeopteryx showed no sign of the structures in modern birds that prevent twisting of the spine during flight. One of least bird-like features of Archaeopteryx was its spinal column and rather small hipbones.
If Archaeopteryx did take to the air, its lack of a large breastbone suggests that it was not a strong flier; perhaps able to fly in short bursts, like modern pheasants. Nonetheless, its wing muscles, however inadequate for sustaining powered flight, were attached to a thick, boomerang-shaped wishbone, which was a bird-like feature.
Although Archaeopteryx is accepted as the earliest bird, it is becoming difficult to see it as much more than another feathered dinosaur. In many ways, Archaeopteryx was just another small, scurrying ground dweller. ~ Canadian evolutionary biologist Gary Kaiser
Microraptor
Another feathered bird-like dinosaur appeared 125 MYA: Microraptor. Like Archaeopteryx, Microraptor had a long tail; another glider among the trees. The long tail may have helped stabilize landings.
Microraptor, like some other early birds, had 4 wings. The legs were feathered for lift. Hindwings were for steering. They allowed Microraptor to reduce its turning radius by 40% and tripled the speed of a turn.
Gliding appears in many kinds of animals, from fish to frogs to squid to squirrels. It offers some of the benefits of flight without excessive energy expenditure or specialized structures. Gliding is an easy route to food on the ground. As any child or cat readily demonstrates, climbing up a tree is a lot easier and faster than climbing down.
Only squirrels go up and down trees with equal ease, owing to a special hip joint that allows hind leg rotation to the point where claws are reversed. Microraptor had no such hips.
The 4-wing model was a transitional step to modern bird architecture. Microraptor went extinct well before the Cretaceous closed.
Avian Pre-Adaptations
Like Dakotaraptor and Microraptor, most dromaeosaurids had fearsomely sharp serrated teeth, flappable wings, and sickle-shaped claws. The recurved claws were used for latching onto prey, preventing their escape. Claw size was relative to prey size.
This strategy is only really needed for prey that are about the same size as the predator: large enough that they might struggle and escape from the feet. Smaller prey are just squeezed to death, but with large prey all the predator can do is hold on and stop it from escaping, then basically just eat it alive. ~ English paleontologist Denver Fowler
It is a modest modification to go from a claw for snagging prey to one for perching: a prehensile pre-adaptation. (A pre-adaptation is a trait that evolved for one purpose which then transforms to another function.)
Flightless, feathered wings were also a pre-adaptation: originally for flapping to stabilize oneself while holding onto struggling meals-to-be. To take flight from there was not such a lofty advance.
Many structures in modern animals originally evolved for quite different purposes. ~ Denver Fowler
○○○
There were a few lineages of avian creatures before the K–Pg event. Only 1 survived: Ornithurae.
Ornithurae specifies the common ancestor to all modern birds. By the time the dinosaurs went down, Ornithurae had already diversified, including the birds that became chickens, ostriches, and ducks.
The Chicxulub impact caused global deforestation. Bereft of trees, arboreal birds could not survive. Only ground-dwelling birds made it through the extinction event.
The forest canopies collapsed. Perching birds went extinct because there were no more perches. ~ American paleoecologist Regan Dunn
The regrowth of forests meant the return of opportunities among the trees, including food and safety. Birds re-evolved the ability to fly.
Modern birds have short tails, as contrasted to Archaeopteryx and other bird-like theropods.
Confuciusornis
Confuciusornis was a crow-sized bird that arose 130 MYA; a transitional genus toward a more modern bird. Though Confuciusornis had slower growth than modern birds, it matured faster than Archaeopteryx. Confuciu-sornis had a pointed toothless beak and no long tail, though some had bladelike tail feathers. Confuciusornis had long wing feathers that were modern in appearance, but its upper body skeleton had not evolved to allow the flapping flight characteristic of modern birds.
Extinction & Evolution
Edentulism – the absence of teeth – evolved convergently among various vertebrates, including turtles, birds, and several lineages of mammals. Birds lost their teeth and went with a better beak ~116 MYA.
Numerous early dinosaur-birds diversified before the Chicxulub asteroid impact. Rather modern-looking birds began to appear over 100 MYA. Though most did not survive the aftermath of the Yucatán big bang, many living avian families flew among the dinosaurs.
The birds that did cross the K–Pg boundary were able to do so because they were small generalists, able to relocate and adjust to different environments. Being able to subside on seeds was probably critical.
The birds that made it quickly adapted to further their flight capabilities, as well as taking advantage of nutrient sources and nesting opportunities. The wings, tail, hips, legs, and feet refined, as did the core parts of the body that power flight.
The most momentous change birds underwent was losing their long bony tails during the early Cretaceous. This freed up their legs to become versatile and adaptable tools.
Avian flight allowed birds to readily populate otherwise remote or difficult biomes. Geographic isolation promoted speciation.
The islands and archipelagoes that dot the western Pacific and Indian oceans provided the most prolific sites for adaptive radiation. The forests of the Andes and Amazon basin were other sites that especially engendered rich diversity, owing to niche habitats that offered opportunities for specialization.
Though birds had long been consumers of plant parts, ~50 MYA flowers managed to entice birds into mutual relations. Bringing birds into the fold as pollinators gave plants an edge on their heavy reliance upon insects.
More than 10,000 avian species have lived since hominins came out of Africa. Extant species are a small fraction since Aurornis appeared.
A confounding facet in following paths of avian descent is convergent evolution. Birds not closely related adaptively evolved selfsame traits. Conversely, birds that look quite distinct may well be relatives.
Except for long claws on their back toes, the meadowlarks of the North American grasslands look like the longclaws of the African grasslands. These distantly related passerines are both ground dwellers of similar size, with streaked backs, yellow underparts, and a bold black V-shaped mark on their chests. The shared features reflect adaptation to a largely identical lifestyle in geographically distinct but identical habitats.
When Darwin passed through the Straits of Magellan in 1838, he was amazed to find birds that looked and behaved just like those he had seen in the North Atlantic. Darwin’s confusion was eminently understandable. Later analysis of the petrels and auks that Darwin saw showed such similar morphology that their classification is easily brought into question.
The same applies to hawks and owls, grebes and loons, swallows, and swifts. Convergent evolution in the numerous families of forest birds, which have been most prolific in speciation, can reduce lineage determinations to guesswork.
Biomolecular analyses offer some promise in filling the enormous gaps between modern birds and their Mesozoic ancestors but determining the lineages of birds is among the most difficult exercise among extant animal classes.
75% of modern bird species live in the tropics. Owing to sheer numbers much niche speciation has occurred there, as avian residence is long-standing. Dramatic climate changes over geological time have been greater in the higher latitudes, but proximity to the poles has little to do with species formation.
Diversification rate does not vary with latitude, though it does by longitude. Bird species abound in the Eastern Hemisphere, but Western Hemisphere birds have diversified faster than those in the east. No single lineage has driven this hemispheric pattern. Instead, scattered bursts of rapid speciation have transpired on a variety of evolutionary branches, likely owing to a variety of causes, including habitat isolation from climate changes.
Pedestrians
Birds are most striking for the variety of adaptations that optimize flight ability: hollow, lightweight bones; a specialized air-sac breathing system; a large breastbone which affords anchoring large, strong flight muscles; and a rib structure which succors superior body support. Nonetheless, for the most part, birds are pedestrians. Only birds of prey earn their living in the air, and only migratory birds spend a great deal of time in flight.
Most birds forage by walking about, either on ground or in the trees. Birds that feed in the water have long legs for wading. Flight simply serves for an express getaway or a quick commute.
Feathers
Feathers give meaning to the word bird. ~ Gary Kaiser
Little of the visible part of a bird is living tissue. Feathers are made of keratin, a tough fibrous protein. For protection during flight, avian eyes are covered in a sheet of keratin.
Feathers evolved from reptilian scales 100 million years before they served any locomotive purpose. Birds still have scales on the lower part of their legs and feet.
Like reptiles (and, later, birds), dinosaurs had highly differentiated color vision (tetrachromacy): much superior to that of humans and other mammals. This led to feathers summing up to something more: plumage, a colorful coating. Insulation and coloration were feathers first purposes.
The membranous wings of bats are effective, whereas the feathered wings of birds are splendidly sophisticated. No other animal has so much non-living tissue that does so much.
The similarity of feathers among both modern birds and their distant ancestors suggests that evolutionary constraints on feathers have always been strong. ~ Gary Kaiser
Flight wears on feathers. Every bird goes through a series of plumage changes during its life.
Besides flight, feathers protect, insulate, and communicate. No other body structure serves such diverse purposes. Plumage often displays a readiness for reproduction, and, in males, acts as advertisement for the act.
Darwin and his followers long assumed that the 1st function of colorful feathers was a dancing display of color for mating: that sexual selection made males put on more sexy plumage. But male birds with multiple mates tend to be drabber than their female counterparts. Male red-winged blackbirds might have up to a dozen mates, but they are less colorful than their consorts.
Avian females have varied between drab and colorful through time. To be less conspicuous to predators, migratory birds tend to be less vivid.
Plumage is important for social signaling and camouflage, not just sexual selection. English naturalist Alfred Russel Wallace, a contemporary of Darwin, thought as much. The pulchritude of avian plumages owes to a confluence of factors.
Ecology and behavior are driving the color of both sexes, and it is not due to sexual selection. Both natural and sexual selection have been influential, but they have generally acted on 2 different axes: sexual selection on an axis of sexual differences and natural selection on both sexes for the type of color (for example, bright or dull). ~ Canadian ethologist Peter Dunn et al
For all the variety of roles, modern avian plumage comes in 4 basic types. Each differs by lifestyle.
Flightless birds make do with basic body coverings of very few feathers with specialized shapes. Nonetheless, these feathers range from the simple hair-like structures on kiwi to fluffy ostrich plumes.
In contrast, flying birds have many specialized feathers, albeit in 2 basic types. Terrestrial birds of all sorts have soft and flexible plumage; typically, a small number of relatively large feathers. Ducks and seabirds are at the other end: firm, densely packed plumage from a copious number of small feathers. Penguins take this to an extreme, with feathers so tightly packed that they create a waterproof surface, much like the scaly covering of ancient reptiles.
Owls can fly almost completely silently. Their wings are broad and curved – ideal for slow gliding – and abundantly veined with velvety down plumage that absorbs sound. The feathers at the edge are serrated, effectively breaking up and smoothing out air turbulence, like a comb slowly untangles knots.
The compound responsible for feathers – keratin – comes in chemical varieties. Families of birds independently developed specific keratins best suited to the purposes to which they are put.
Coloration
Avian coloration represents a compromise between the brightness that fires courtship display and the dullness needed for camouflage. Acuteness of color vision and peak coloration commonly go together in birds.
Feather colors are produced by various pigments deposited in feather barbs and barbules, and by structural tweaks at the feather surface. Most greens and blues result from structural alignments. Pigments produce the other colors.
There are 2 kinds of structural color: iridescent and non-iridescent. Both result from the microscopic structures of feathers which reflect only certain light wavelengths. Iridescent colors change with viewing angle. Iridescence usually only occurs on body feathers, rather than those used to fly, as the affects which give rise to iridescence weaken the feather. Some iridescent colors result from reflections at the interface of a feather’s melanin (pigment) granules and keratin layers.
Non-iridescent feathers manage their color display via tiny air pockets (vacuoles) within barbs which scatter incoming light. Such scattering produces the blues on some birds, including bluebirds.
The most peculiar aspect of color is the relative wavefront coherence by which it is generated. The blue sky is produced by incoherent interference of reflected light waves. By contrast, the blue of bird feathers occurs when light is reflected in phase, via coherent interference.
In feathers producing color structurally, color purity reflects the orderliness of the material employed. Iridescence arises from tightly packed arrays.
The colors and patterns of birds can vary as a function of sex, age, nutrition, and season. While some birds are similarly colored regardless of sex, plumage dimorphism is common. In some birds, such as gulls, color patterns differ each year until adulthood (which for gulls is 4–5 years).
In birds with elaborate courtship displays, the displayer is brightly adorned, while the watcher is typically dull-colored. Males are typically the exhibitionist, but in phalaropes (a shorebird), dull-colored males incubate the eggs whereas females perform courtship displays.
Many brightly colored birds shed their florid display at the end of breeding season, replacing it with a dull winter coat. Prominent adornment feathers used for courtship may also be lost.
Wings & Flight
There is a surprising diversity in avian wings, the shape of which affects how a bird flies and how far it can fly at a stretch. Whereas round wings aid maneuverability, long wings facilitate sustained flight. Wings further reflect lifestyle in how birds use their wings: to flap, glide, or dive.
Birds that migrate long distances have long, thin wings. Most such birds spend part of their lives in the higher latitudes. Bird wings generally become shorter and rounder the closer to the equator a bird lives. Such wings abet flying in forests and amid thick brush, when ready lift and maneuverability are essential.
Like the forelimbs of amphibians, reptiles, and mammals, avian wings are tripartite: an upper arm (brachium), forearm (antebrachium), and hand (manus). The upper arm and forearm are elongated to afford the attachment of the muscles and flight feathers needed to fly.
The ratio of total wing area to body weight – wing loading – determines the ease with which a bird can fly. As avian wings are more curved above than below, air flows faster across the top as a bird flies. This lessens the pressure above the wing (Bernoulli’s principle), providing the lift which keeps a bird aloft.
Flight is a matter of counteracting weight with lift and drag with thrust. Avian wings and bodies are superbly designed for what birds can do on the wing.
The feathers on the wing tips can be compared to the spread fingers of a hand. The wing tip generates several small air vortices instead of one large vortex. It requires more energy and costs more to lift off when only one large wing tip vortex is generated. ~ Swedish evolutionary ecologist Anders Hedenström
For forest birds, the most important aspect of flight is the ease with which wings can deliver lift (as opposed to thrust, which only becomes significant in sustaining flight). The emphasis on lift meant evolutionary measures to reduce weight. The most common approach has been to lessen overall density while increasing volume: minimal bodies amid an impressive halo of feathers. Although a lightweight feathery cloud reduces density, it does so at a cost increasing air resistance (drag), which ups the energetic cost of fast or sustained flight.
Lower density comes in handy in a fall. Perching birds such as the wood duck or bufflehead, which roost in trees, depend upon the fluff that protects their young when they drop from the nest. Some fledglings must drop 10–20 meters when they mere balls of down, long before they can fly.
Weight is partly reduced via temporary reductions in soft tissues. Reproductive organs do not grow until a bird reaches sexual maturity, and then shrivel after breeding season. There is even evidence that part of a songbird’s brain shrinks when such birds are not singing.
Avian skeletons have been more permanently trimmed via simplification, which often involves fusion. Articulating joints are relatively heavy, as they contain fluids and pads of connective tissue. A reduced number of such joints is especially widespread among forest birds. Many avian fused joints are in the head, replaced by flexion areas of exceptionally thin bone. The best known of these flexion areas connects the upper beak to the braincase.
There are exceptions. Parrots need a strong skull to peel tough-skinned fruits, and so have retained many independent bones in their heads. Many other birds ingest food whole, and so may dispense with unneeded flexion complexities.
Bones
All terrestrial vertebrates rely on bone for bodily rigidity, to anchor muscles, and to protect vital organs. Bone is not the only evolutionary possibility.
Because it can be rigid, tough, or flexible as need be, chitin is the select material of structural integrity in arthropods. But chitin has some severe limitations. A glucose derivative, chitin is a non-living material: spun fibers bound by cross-linked proteins. Chitin cannot be stretched or reabsorbed to afford growth. Its employment obliges arthropods to repeatedly undergo complicated shedding regimes during development.
Made of protein fibers, keratin is somewhat like chitin as a structural material. Keratin was an innovation for vertebrates, and is used for nails, scales, horns, beaks, claws, hooves, and hair. Unlike sugar-based chitin, protein-based keratin can grow continuously, and so is employed for structures which naturally wear away.
For supporting a body, especially on land, bone cannot be beat. Bone is a physiologically active tissue that can change size and shape during development. The assembly of bones as skeletons allows flexible and efficient motion, as each bone can be stiff enough to resist the local forces of compression, shearing, or bending.
The schematics of skeletons are generally consistent among vertebrates, including the arrangement of the skull and lower jaw, the string of vertebrate that compose the spine, and limbs that have a single large bone near the body, a middle section comprising a pair of bones, and an outer instrument that is a fan of digits. Most the major body plan innovations among vertebrates involve variations in the shapes of these structures. There are however significant distinctions between birds and those animals unable to take to the air.
Whether on land or in the water, an animal’s skeleton maintains body shape during movement. In terrestrial animals, bones keep a body off the ground: a constant gird against gravity. Bird bones must do something more: allow lift off the ground. To accomplish this, the avian skeleton trades off sturdiness for lightness.
With fewer bones than a reptile or a mammal, a bird is uniquely built to withstand the forces of flight. ~ American biologist Joanna Burger
Pedestrian birds like the ostrich are built with bones like mammals, but a bird that flies resembles an assemblage of springs and levers under tension rather than a sturdy framework.
Early dinosaurs had as many as 9 wrist bones. The evolution of bird wings improved economy on that account, as bird wrists have only 4 bones.
Bird bones reflect a commitment to flight, most notably a centralization of muscles near the body’s core. Centralization requires that some bones provide broad surfaces for attaching large muscles. Centralization also results in bones with highly sculpted surfaces for guiding the bunches of tendons that frequently cross more than 1 joint. The wicker basket the comprises a bird’s skeleton holds a more much elaborate architecture than mammals.
Bird bones are best known for being hollow, but the savings in weight of hollow bones is only 8–13% less than a comparable marrow-filled bone. The hollow construction instead sacrifices flexural (bending) strength for high torsional strength: the ability to withstand a twisting load.
Further, the hollow core of avian bones is not empty space. Bird bones are instead chambers put to several uses. Most are filled with air that connect to the air sacs that are part of a bird’s unique respiratory system.
Other bones are used for storage. Some hold the calcium supply for producing eggshells. Some have the fat-rich marrow ubiquitous in mammal (and dinosaur) bones.
Muscle & Tendons
In mammals, the muscles that move limbs are distributed throughout the body, close to the joints they move. There is ample room on the limbs for a copious blood supply, and even fat storage.
In birds, the muscles are remote from the joints. Limbs are sticks. Even the fleshier wings have only enough tissue to manipulate the feathers in flight. No fat is stored there. Blood vessels and nerves are few.
Bird muscles are collected close to the body’s core, far from the action. Even the muscles that curl the toes are mounted near the hip. Long tendons carry the strength that the muscles provide.
Tendons from muscles along the breastbone flap the wings of birds. Tendons along the backbone twitch the stiff feathers of the tail (rectrices), which are an essential accoutrement in steering and braking during flight.
Lungs
Modern-day amniotes (reptiles, birds, mammals) have 1 of 2 basic lung types. Both derived from a single Carboniferous reptile with simple sac-like lungs.
The alveolar lungs of mammals have millions of highly vascularized, spherical sacs called alveoli in which air flows in and out. Breathing is accomplished by expansion and contraction of the rib cage and diaphragm. The many alveoli in this type of lung makes for efficient oxygen acquisition. Alveoli efficiency is offset by a 2-way breathing tube.
The most inefficient way to breathe is the mammalian way: inhalation and exhalation through the same tube into the lungs. The inefficiency comes from the disorder of the gas molecules as one exhalation finishes and one inhalation starts. In any sort of rapid breathing, there is a chaotic collision of exhaled air trying to get out before inhalation begins – and quite often the same gas molecules, including volumes of air with more CO2 and less O2, are sucked back in. ~ Peter Ward and Joe Kirschvink
In contrast to alveolar lungs, the septate lungs of reptiles and birds is a single large sac, partitioned by septa: bladelike sheets. Septa increase the surface area for respiratory exchange.
Septate lungs are not elastic. Snakes and lizards move ribs to draw air in. The side-to-side swaying of lizard locomotion inhibits lung cavity expansion, so lizards do not breathe while moving; hence lizard movements are in bursts.
In contrast to the relative uniformity of alveolar respiration, there are many variations of the septate lung design and ventilation. Crocodiles have a septate lung and a diaphragm, albeit a diaphragm different from that of mammals. Snakes, lizards, and birds have no diaphragm.
Birds have their own unique septate lung system. Avian lungs are small and somewhat rigid. They do not expand and contract on each breath. Nonetheless, the rib cage is very much involved in respiration.
Birds have additional air sacs attached to their lungs. When a bird inspires air, the intake flows into a series of these air-sac appendages. From there the air passes one way into the lung tissue. The air is exhaled out the lungs.
One-way air flow across the lungs affords a countercurrent setup: air passes one way, while blood vessel flow goes the other way. This countercurrent creates a more efficient system for oxygen extraction and carbon dioxide venting than is possible in dead-end alveolar lungs.
At sea level, bird respiration is 33% more efficient than that of mammals. At 1,500 meters, birds breath 200% better than a mammal ever could: hence geese can fly over the Himalayas at an altitude that would kill a human.
The unique avian respiratory system descended from saurischian dinosaurs. There is no indication that ornithischians had this air-sac system.
Manipulation
The evolution of the beak was a seminal step for modern birds. ~ evolutionary zoologist Arkhat Abzhanov
With forelimbs for flight and hind limbs for walking, birds must rely upon their beaks for manipulative tasks. In effect, a bird’s neck is like an arm, and its beak the equivalent of a hand.
To facilitate beak use, birds have exceptionally long, flexible necks: assembled from numerous parts, supported by a complex array of muscles, tendons, and ligaments. Bird necks are like those of the dinosaurs from which they descended.
Birds often use rapid movements to capture prey. A heavy head would be a strain on the neck and spinal cord.
Dinosaurs had lightweight heads. Birds extend this trend to an extreme. The size of bird brains corresponds with the physiological requirement of apt manipulability of the beak. In other words, avian brain size has nothing to do with cognitive ability. Many bright birds have tiny brains. (Where brains are resident in organisms, they are mere artifacts of the intelligence system, which is energy-based. Physicality is always a crude correlate of coherent energy forms.)
The bones in a bird’s skull are mostly paper thin, providing little protection. Even the jaws are lightweight, despite their crucial duties.
The shape of a beak tells a poignant story of each bird’s evolution and survival. ~ American ornithologist Noah Strycker
The earliest birds had teeth specialized to their diet, but they economically lost their teeth early on. Jaw muscles moved closer to the body core.
Birds use the beak for literally everything. They evolved a versatile tool not just for getting food, but also to accomplish many other tasks. ~ Spanish paleobiologist Jesús Marugán-Lobón
Of all avian traits, the beak appears the most flexible in adaptivity. Though the bill often contributes to it owner’s repertoire of displays and communiques, its shape reflects feeding habits.
In birds of prey such as eagles and falcons, the shapes of the skulls change in a predictable way. The shape of the beak is linked to the shape of the skull, and these birds can’t change one without changing the other. Being able to break this constraint — letting the beak evolve independently from the braincase, may have been a key factor in enabling the rapid and explosive evolution of the thousands of species of songbirds. ~ English evolutionary ornithologist Jen Bright
The finches of the Galápagos Islands were seminal in Darwin’s speculations on evolution. The islands undergo cyclical wetter and drier climates. The seeds which the finches feed upon get larger in moist times and shrivel during aridity. Via epigenetic tailoring, finch beak lengths change to better eat the extant seeds.
The connection between beak shapes and feeding ecology in birds is weaker and more complex than has been historically expected. While there is definitely a relationship there, many species with similarly shaped beaks forage in entirely different ways and on entirely different kinds of food. ~ Spanish paleobiologist Guillermo Navalón
Bird bills vary vastly in size, shape, and strength. Among other tasks, beaks are adapted for tearing meat (hawks), grasping fish (terns), cracking seeds (finches) probing crevices (woodpeckers), sussing what is in the sand (sandpipers), and straining microscopic food from mud (flamingos).
Herons, cranes, and storks have simple, elongated beaks for picking up small prey, but bill length is important if a bird is to hold struggling prey away from the eyes, which must be safeguarded.
Some hummingbird beaks are adapted both for nectar collection efficiency and fighting.
Instead of feeding on a particular flower shape very well, some birds try to exclude everybody from a patch of flowers, even though they can’t feed as well on them as hummingbirds without bill weapons. If you are good enough at keeping your competitors away, then it doesn’t matter how well you use the resources in the flowers you are defending – you have them all to yourself. ~ Columbian evolutionary zoologist Alejandro Rico-Guevara
Even the most robust beaks have sophisticated structures and specializations. Near the tip are spaces for tiny taste organs and other sensory receptors which help sort out edible from inedible items. These must be kept clear. Birds frequently trim their bills by wiping the edges against some hard object.
In most birds, the horny beak is no more complicated than a pair of chopsticks. Like chopsticks, the beak is useful only because it is manipulated by a sophisticated “hand.” Whereas mammal jaws have 2 robust bones that work against each other, birds have 9 or more small bones that sometimes work together and sometimes in opposition.
Special hinges allow the upper jaw to rotate against the skull, and the arms of the lower jaw can flex outward near their midpoints. Both movements increase gape and enable a bird to cope with inconveniently shaped foods. Some bones in the anterior jaw are not attached to muscles, instead acting as levers or pushrods, bound by ligaments to bones further back in the mouth that are muscle-bound. The working of a bird’s jaw has more in common with the shuttling of a loom than with the grip of the human hand.
Bills are usually dull-colored, although some birds have colorful beaks which are used in courtship displays. Even closely related birds may have quite different beaks.
Birds have a very non-mammalian tongue, in that it actively handles food. A bird’s tongue is neither soft nor fleshy: it has its own internal skeleton, termed the hyoid apparatus. Birds employ their tongues to secure, manipulate, taste, and swallow food. If the avian jaw functions as fingers, the tongue is the thumb. Like beaks, tongues vary in length and shape.
Until the early 20th century, birds were thought to lack taste buds. Our taste buds are on our tongues. Avian taste buds are on the base of the tongue and on the roof or floor of the mouth. Whereas humans have over 10,000 taste buds, birds may have anywhere from 2 dozen (chickens) to a few hundred (parrots). Yet birds differentiate tastes quite well, with sensitivities corresponding with diet. Calidrids, a group of shorebirds, recognize where worms have been crawling in sand. Hummingbirds can discriminate among sugars and their concentrations.
Even the most robustly jawed birds can only initiate food processing in their mouths. Some species deftly crack shells off seeds, or tear chunks from prey. Most merely can orient their food intake for comfortable swallowing. Chewing is out of the question.
Even the saliva produced by some birds lack digestive enzymes. Both chemical and mechanical reduction to digestion are delayed until the food enters the muscular foregut, deep within the body cavity.
Even in the gut, birds, like their evolutionary predecessors, lack any grinding surface. Hence birds swallow grit or small pebbles for mechanical reduction. These digestive aids may not be lighter than a set of teeth, but they are in body core, not in the head.
Woodpecker Beaks
Woodpecker beaks absorb tremendous shocks when battering trees to get to grubs within. Like most birds, a woodpecker’s outer beak layer is a keratin sheath, arranged in a scale pattern, with defined edges between each scale. The scale edges of woodpecker beaks follow a zigzag pattern. Other pecking birds have a straight scale edges. The zigzag deflects compressing forces as a beak hits wood. Woodpecker beak scales are thinner and more elongated, affording greater scale sliding, which serves for shock absorption.
The middle “foam” layer of a woodpecker beak is more porous than in other pecking birds. This directs the energy of impacts to other parts of the woodpecker’s head that are better equipped to absorb shock. All told, woodpecker beaks have an unimaginably sophisticated design for battering wood while not damaging a fragile skull.
○○○
Beaks are not the only manipulative tool birds have. The feet, and especially the claws at the end of the toes, are adapted to lifestyle. Ground dwellers often have elongated hind claws which help from sinking into soft sand or mud. Tree climbers have curved claws that can cling to rough bark and climb vertical surfaces. Birds that grasp and tear at their prey have strong and highly curved claws.
◊ ◊ ◊
Long bird necks are uncommon because they are vulnerable to attack and readily radiate body heat. Hence, birds with long necks have a protective posture: fold the neck and tuck the head into the body. This pulls the neck muscles close to the warmth of the body core without sacrificing outreach as need be.
In flight, plumage covers the neck and smooths the angular contours of the shoulders, reducing drag via unbroken airflow. Long-necked migratory birds fly with the neck extended to optimize aerodynamics.
Flightless Birds
For 10 million years after the K–Pg extinction event mammals remained small. This ecological vacuum gave a window of opportunity for the evolution of large herbivores. Birds that had flown to occupy the islands of Madagascar (from Africa), Mauritius (from India) and New Zealand (from Australia) upsized, as did a few terrestrial birds on continental landmasses.
Putting on weight put getting off the ground out of reach. Wings withered. So evolved the large flightless birds: the ostrich in Africa, elephant birds in Madagascar, the dodo in Mauritius, the emu and cassowary in Australia and New Guinea, the moa and kiwi in New Zealand, and the rhea in South America.
