In many respects, birds’ anatomy is comparable to human anatomy. The differences fascinate for two reasons. First, birds’ features have assured avian presence on our ever changing planet for millions of years. Second, their unique combination produces the miracle of flight!
Generally, birds’ skeletons are similar to those of other vertebrates. They have a long, central spine with attachments. Where they differ, however, is in skeletal characteristics associated with flight. Whereas most animals have heavier bones, flying birds have thin-walled, hollow bones with internal struts for support. This pneumatic bone structure makes them lighter. Flightless and diving birds have solid bones.
In addition, birds have longer necks with freely moving vertebrae so they can catch food and preen. Whereas humans and other mammals have seven neck vertebrae, even short-necked birds have 14, and swans have 25!
A bird's skull has large nostrils and eye sockets. Today's birds have toothless upper and lower mandibles (jawbones) that define light, strong bills. Beneath the skull, cervical vertebrae define the neck. The upper backbone is short in comparison with most other animals. The pectoral girdle includes the shoulder blades and the wishbone, which attaches wing joints to the breastbone.
In the back, the lower region is fused to a strong pelvic girdle composed of three bones. Birds no longer have true tails. The remainder of the backbone extends below the lower back, culminating in a stump to which feathers attach.
Flattened ribs join the backbone to the large breastbone in the front which protects the organs from impact. Except in flightless birds, the breastbone is “keeled” to anchor flight muscles.
The major upper wing bone fits into a socket in the shoulder blade. Below that is a thicker bone paired with a thinner bone, corresponding to the human forearm. A “thumb” extends off the joint below the thinner bone.
Comparing birds’ legs to humans’ only confuses matters. Birds’ legs look as if they bend forward at the “knee,” but that knee is actually a “heel.” Like human legs, bird legs have a thigh bone and a knee, both hidden in feathers. Below the knee are two bones, a larger and a smaller. The vertical bone beneath corresponds to a human foot's instep and is called the tarsus. Birds actually stand on their toes!
Most birds’ feet have four toes, three facing forward and a hind toe facing backward. In most birds the hind toe is on the same level as the other toes. However, in birds that spend most of their time on the ground (like pheasants and cranes), the hind toe is elevated. A few birds have three toes; an ostrich has only two. (See also the section below on “Feet and Legs.”)
The Nervous and Endocrine Systems
The nervous and endocrine systems are the bird body's command mechanisms. They work together for the bird's survival. The nervous system relays commands through neurons, the endrocrine system through hormones. Messages from the nervous system produce instant results. Messages transmitted by the endocrine system cause slower changes.
Despite the disparaging term, “bird brains” are actually larger and more adept than those of any other vertebrates except mammals. They are keenly functioning, particularly in receiving and processing messages from sense organs. These responses streamline survival actions such as decision making regarding fight or this flight. Unlike mammals, birds have a forebrain which neurologists believe is the seat of their intelligence. It seems to control their ability to strategize, recall food sources, use tools, and outwit predators.
The larger the forebrain the higher the avian I.Q. Corvids (crows, ravens, jays, and magpies) and Psitticines (parrots, macaws, parakeets, and lories) have brain-to-body ratios equaling that of dolphins and almost matching humans’. They are known to be cunning in most respects and have a remarkable genius for tool use (Chicago Tribune, January 18, 1996).
The brain and spinal cord together comprise the central nervous system. Ganglia extend into the peripheral nervous system. Neurons (nerve cells and their fibers) receive and decipher input from both inside and outside the body. They then relay electrochemical impulses that produce the immediate action required for survival. Birds’ brains closely resemble those of reptiles and amphibians. Birds of superior intelligence, such as parrots and crows, have larger brains. Recent studies at the University of Washington proved that male birds living with females have brains 15 to 20 percent larger than birds living alone or with other males (Scientific American, January 1998).
The endocrine system also sends messages, but these are carried by chemical substances called hormones. There are thirteen ductless endocrine glands in the bird's body. These glands secrete hormones that empty directly into the bloodstream and circulate throughout the entire body. All cell membranes have receptors to one or more hormones. The binding of a hormone to a cell receptor initiates particular changes in the cell. With the exception of the adrenal gland, they promote not immediate action but gradual modifications such as sexual and seasonal changes, metabolism, and growth.
The pituitary is the most important gland, secreting hormones that stimulate secretion of other glands, such as the thyroid, adrenals, and gonads. The thyroid gland is central to controlling a bird's metabolic rate and body temperature, and may help initiate migratory behavior. Like the pituitary, it also impacts feather development and molting. The thyroid is located in the throat.
Birds have a special oil gland, called the “pip,” just above the base of the tail. Birds use oil secretions to preen their feathers. These secretions help keep the bird both waterproof and bouyant.
Bursa of Fabricius
This is not some exotic island or a piece of rare coinage, but rather a little glandular pouch in the upper wall of a bird's cloaca near the vent. In young birds it is open and deep. Named for Johann Fabricius, the bursa of fabricius is part of the immune system. It produces leucocytes in young birds that seem no longer necessary once the bird matures. It atrophies by adulthood. Since it closes or disappears with age, experts can determine a bird's age by its size. This method is not used on small birds because they are just too tiny.
The Muscular System
In birds, muscles move bones, skin, and feathers. Muscle tissue is composed of cells that form fibers. When the muscle tissue contracts, it produces movement in the body.
A bird's most massive muscles power flight. Muscles located on each side of the breastbone keel extend outward. Attached to the long bone of the wing, they propel the wings’ downward motion. Muscles over the shoulder blades are the raising muscles. Smaller muscles in the wing change the configuration of the feathers and wing joints.
In the leg, muscles surround the larger bone. The tarsus has no true muscles, but rather numerous pulleys called flexus tendons. These automatically tighten birds’ toes around a perch so they can relax and sleep without falling off. Rather than exert to hold on, as a human would, a bird has to exert to move away from the perch.
The Circulatory System
A bird's circulatory system carries oxygen, nutrients, and hormones through every part of the body. Blood also regulates tissue water content, immune response, and body temperature.
A four-chambered heart, arteries, veins, capillaries, blood, and the lymphatic system comprise a bird's circulatory system. The heart keeps the blood moving, pumping it through the “irrigation” system—arteries to veins and capillaries. Birds’ hearts have two ventricles, just like humans’. While the left ventricle contracts to pump blood that has been freshly oxygenated by the lungs, the right ventricle contracts to route deoxygenated blood back to the lungs.
Like human blood, birds’ blood consists of plasma and corpuscles. Red blood cells form in bone marrow and, in passerines (songbirds), in the spleen and liver. White blood cells form in the spleen, liver, kidneys, pancreas, and bursa of fabricius (see sidebar). Its red blood cells transport oxygen and carbon dioxide between the lungs and tissue cells.
Birds’ circulation concentrates in the body and wings. Not much blood circulates in the tarsus and toe areas. This helps birds conserve heat, but sometimes leads to frozen toes.
The Respiratory System
Birds’ respiratory system is the most efficient of all vertebrate animals. Birds need continual air flow to sustain flights. Hence, their respiratory system differs from the “in and out” system of most other animals. This remarkable system allows birds to maneuver acrobatically at altitudes at which most animals would barely survive.
Like all vertebrates, when birds inhale air, the oxygen in air is acquired by the blood. Body cells use the oxygen to burn digested food for fuel and to maintain body temperature. Carbon dioxide, a waste product of this burning, moves through the blood to the lungs, where it is then exhaled.
A bird's breathing tissues—nostrils, cere, sinuses, choana, trachea, lungs, and air sacs—can comprise more than one-fifth of its body volume. Air moves in through the nostrils, where it is filtered for impurities by the sinuses and warmed. The choana, a slit in the roof of the mouth, lets air pass into the pharynx and through the glottis into the larynx. From there, it descends to the syrinx, the voice-making apparatus in the lower part of the windpipe. Air divides into the bronchi and passes down into the lungs.
Each of a bird's two lungs have from six to fourteen little balloonlike air sacs appended to them. These extend from the lungs into other regions in the body. Air sacs help reduce a bird's body heat. In some water birds, the air sacs add buoyancy. In diving water birds, the sacs are smaller so the birds can stay under water. Some male birds divert air through cervical air sacs to inflate neck bladders as part of their courting behavior. Occasionally, cervical air sacs can contribute to sound making.
Song and Sound Making
The syrinx, located at the base of a bird's throat, is a resonating box controlled by muscles that alter its membranes’ position. This motion changes the pitch of the bird's song. Air pressure exiting its lungs varies the intensity of the bird's volume. The windpipe functions as a kind of resonator. Many ornithologists believe there is a corollary between long windpipes and deep tones. The more complicated a species’ song, the more syringeal muscles it usually has. Songbirds have up to nine. Storks have no functioning syringeal muscles. Turkey vultures do not even have a syrinx.
Birds that gulp a lot of water with their food—cormorants, anhingas, herons, grebes, storks, and some fowl—have a little third stomach chamber in addition to their stomach and gizzard. Sometimes it contains feathers. It appears that the pyloric stomach filters the water.
The Digestive and Excretory Systems
Birds have an extraordinarily high metabolic rate that corresponds to the immense energy demands of flight. Small birds eat as much as 25 percent of their weight daily, and their food-processing apparatus has the immense task of turning that into expendable fuel. A bird's digestive system ingests and digests food, then transfers nutrients to the blood and takes waste products out of the body.
A skin pocket in the throat of a bird such as a pelican is called a gular pouch, a soft, elastic pouch that holds partly digested fish on which the young are fed. Fully stretched, a pelican's gular pouch can hold as much as three gallons.
If birds had teeth they could chew food before delivering mouthfuls to the esophagus and stomach. But birds cannot linger over their meals a bite at a time as we do, because they would be too vulnerable to predators. A bird's crop allows it to swallow food whole quickly and then fly away to safety. The crop is an enlargement of the esophagus and serves as a temporary storage area for food. It can continually deliver small amounts of ingested food to the stomach where it is rapidly processed. Some birds swallow large prey whole and store it in their crop until it is softened. Birds that have little reason for storing food, such as seed-eating songbirds, may not have fully developed crops. Some birds use the crop in courtship as a sound maker. Pigeons and doves produce liquid food for their young in their crop.
Without teeth, birds have developed an amazing mortarlike organ called the gizzard. Combined with sand or pebbles that the bird swallows, the gizzard can crush hard seeds, nuts, grains, and shells. During the eighteenth century, several curious European scientists conducted experiments on turkeys. A turkey gizzard crushed tubes of tin plate within twenty-four hours. The same type of tubes required 437 pounds of pressure to flatten them in a vise. Another experiment showed a turkey gizzard grinding up twelve steel needles in a day and a half! (Audubon Society Encyclopedia, p. 442) Birds of prey regurgitate indigestibles such as fur, feathers, teeth, and scales in the form of pellets. Pellet forming scours the throat.
Differences Between Sexes
It's a Boy! or It's a Girl!
Often one can distinguish between bird genders based on size and plumage or even eye and bill color. In species in which only the female incubates, one can check for an incubation or brood patch [slightly unfeathered breast). Male ducks, geese, and swans have penises within the cloaca; domestic chickens, turkeys, pheasants, and herons have tiny vestigial penises.
Male and female birds look the same when they hatch. However, in many species, males and females soon begin to exhibit differences in color, plumage, size, and, in some species, combs and spurs. As adults these discrepancies extend into song and mating behavior. In some, the contrast between genders is more pronounced than in any other vertebrate.
In many species, females are duller colored and therefore better camouflaged. Only in a few is the female's plumage more vibrant. Whether seasonal or permanent, these disparities help reinforce breeding only within species.
In most species, males are bigger than females and compete aggressively for mates. Less frequently—in birds of prey, frigatebirds, and phalaropes, for example—females are larger and more aggressive in mating.