TEXTâO A knowledge of the anatomical structure of birds (or any animal) is useful for at least two reasons: it affords a greater understanding of behavior, and it also provides the groundwork for a classification system based on physical similarities among living species and fossil forms.╥Anatomy╙ is the structure of an animal and ╥morphology╙ is the rigorous study of anatomy. Because birds and humans are both vertebrates and share some basic similarities in body plan, we can relate the structure of birds to our own anatomy. This is helpful in understanding, for example, that the ╥alula╙ of birds (a small but significant feathered appendage near the end of the wing) can be thought of as the counterpart to our thumb. Birds also have specialized anatomical features not found in humans, of course, and we need to learn another vocabulary for describing these body parts and areas.We can further subdivide internal morphology: Osteology is the study of bones and the skeletal system. Myology is the study of muscles and their systems. Neurology is the study of nervous and sensory systems. In this discussion we will consider internal anatomy by selected functional systems: skeleto-muscular, sound production, respiratory, and sensory.INTERNAL ANATOMYSkeletonThe skeleton, or skeletal system, is comprised of both hard calcareous bones and tough, springy cartilaginous bones. Bones provide the attachment sites for muscles and act as levers, performing a vital role in all bird movements. Bony structures also protect the bird's vital parts, such as the heart and brain.The avian skeleton is highly specialized for strength and lightness, and has evolved striking adaptations for flight, swimming, walking, and running. Its lightness and rigidity depend on slender, hollow, air-filled bones, and added strength from the fusion of different bones. This fusion reduces the number of parts and simplifies the overall system. Each section of the skeleton has a particular specialization. The main divisions, often considered separately, include: skull; vertebral column and ribs (with pelvic girdle); sternum (breastbone); pectoral girdle; and bones of the wings and legs.SkullA bird skull is relatively light and generally consists of a rounded brain case, roof of mouth, jaws, and palate. The brain case includes large eye sockets, or ╥orbits╙ surrounding most of the eyeballs, as well as the thin overlying bony plates called ╥sclerotic rings,╙ that protect and strengthen the eyeballs. The bill, or beak (without teeth, in modern birds), is distinctive and is an extension of the bony jaw. An especially good example of the relationship between behavior and morphology is seen in woodpeckers╒ skulls. The bones of both skull and orbits are thickened to protect from the shock of hammering. Special surfaces are devoted to the attachment of muscles for the very elongated tongue.Vertebral Column and Ribs The remarkable flexibility of a bird╒s neck derives from the high number of cervical, or neck, vertebrae, and to the odd shape of the articulating (connecting) surfaces of the vertebrae. These allow a bird great movement in lowering, raising, or moving its neck from side to side. Humans have only 7 neck vertebrae, but some birds (common pigeon, for example) have 14; swans, 25; geese, 19; ducks, with shorter necks, 16 or 17; and hummingbirds, surprisingly, 14 or 15.The thoracic (chest) skeletal region consists of the vertebrae, ribs, and sternum. The last thoracic vertebra fuses with the thirteen below it to form the ╥synsacrum,╙ a sturdy formation anchoring the legs and tail muscles. Sternum, or BreastboneThe evolution of the breast or pectoral muscles that drive the wings of a bird was paralleled by an increase in the size of the sternum in all modern flying birds. It is a somewhat flattened, plate-like bone that protects the chest and part of the belly against physical blows. It has a deep keel (╥carina╙) that strengthens the sternum and gives extra space for the attachment of the flight muscles.Pectoral GirdleThe pectoral girdle, an arch of bones including the scapulas, coracoids, and clavicles, supports the wings and provides critical bracing and support in flight, while still allowing needed flexibility. It is a tripod of strong bones, among them the heaviest bones in the bird╒s body. The two upright coracoids, in particular, link the forward end of the sternum to the wing assembly. Attached to and suspended in front of the two coracoids is the ╥furcula,╙ or wish-bone. Its two bones are the clavicles--one on the right, one on the left--pointing downward. They approximate the equivalent of the human collarbone. The angle, or spread, of these two clavicles is generally widest in birds of strong flight, with each clavicle serving as a strut to brace the wings apart.WingsAt the top of each coracoid (discussed above), at the point of attachment of the scapulas (shoulder blades), is a socket called the glenoid cavity, which is roughly equivalent to the human shoulder socket. The basal part of the wing bone, or ╥humerus,╙ fits into this cavity. The humerus is comparable to that section of a human arm between the shoulders and elbow. The next section outward, or downward, in the wing of a bird, and comparable to a human forearm, is composed of two bones, the radius and the ulna. The ulna, the heavier of the two forearm bones, bears the secondary flight feathers. The next section of the wing outward, called the ╥wrist,╙ along with the reduced ╥fingers╙ of the ╥hand╙ part of the wing, bears the primary flight feathers and the feathers of the alula, the ╥thumb╙ of a bird.Muscular SystemMuscles are bundles or sheets of specialized cells called muscle fibers. They are held together and surrounded by sheets of connective tissue called ╥fascia.╙ About 175 different muscles, most of which are paired, have been described in birds. Vertebrate animals have three types of muscle: 1) Skeletal muscles are so named because at least one end attaches to one or more bones of the skeleton. They are made up of striated muscle cells under conscious control and so they are also called voluntary muscles. 2) Smooth muscle fibers, under involuntary nervous control, comprise the muscular walls of arteries and the digestive system.3) Cardiac (heart) muscle has its own inner rhythmicity and beats, or contracts and relaxes, without stimulation of the nervous system. It is said that ╥a skeletal muscle can only pull; it cannot push.╙ For each pair or group of muscles that produces a certain movement--for example, bending or drawing in the wing by contracting the muscles--another pair or group of muscles on the other side of the bones can produce the opposite movement when the bird extends or straightens its wing.Muscles of birds have been grouped into areas of function and location: muscles of the skull and jaws; orbit and ear; tongue, trachea, and syrinx; upper limb (wing); lower limb (leg); vertebral column; and body wall. At least four of these muscle groups are important characters used in the classification of birds--syrinx, jaws, and the two limbs.Voice and Sound-MakingA significant and obvious part of bird behavior involves the production of sound. In turn, such behavior depends on complex anatomy. The human voice box (larynx) and that of other mammals is in the upper part of the trachea (windpipe). Sound is made when the vocal cords are moved by an exhaled breath. Birds also have a larynx at the upper extremity of the windpipe, but it lacks vocal cords and has little or no part in producing voice sounds.A bird╒s voice box is the syrinx, in the lower part of the windpipe at the point where it divides into two bronchi, each of which passes directly to the corresponding left and right lung. Of all animals, only birds have a syrinx. Its position and weight distribution lower in the body, near the lungs rather than the throat, helps the aerodynamic balance of a flying bird.The box-like, membranous or bony syrinx typical of most birds is a resonating chamber strapped with elastic vibrating membranes, the whole of which is controlled by specialized muscles. These muscles, called intrinsic muscles of the trachea and syrinx, can alternate the tension and position of the membranes and thus change the pitch (frequency) of a bird's voice. By changing the air pressure from its lungs, a bird can vary the intensity of volume of its voice, and to some extent, its pitch. Typically, birds with the most syringeal muscles produce the more complex songs or calls. Pigeons have a single pair of syringeal muscles; most songbirds have five to nine pairs; and versatile vocalists such as crows, catbirds, and starlings have seven to nine pairs. Turkey Vultures lack a syrinx and the structures associated with it and storks have no functioning syringeal muscles. Such birds are usually called ╥voiceless,╙ but they can nevertheless grunt, hiss, or make booming sounds.The paired air sacs in the necks of certain game birds, such as Prairie-chickens and Sharp-tailed Grouse, act as resonating chambers when the males utter their tooting, booming courtship calls in spring. Trumpeter Swans and Whooping Cranes, while belonging to two entirely different families, both have enormously long windpipes that produce deep, trombone-like sounds. Through convergent evolution, each has developed an extraordinarily long trachea, part of which is coiled within the keel of the breastbone.Respiratory SystemLungsBirds have a respiratory system that is the most efficient known among all vertebrate animals. The lungs are the center of a bird╒s breathing apparatus, with an intricate network of ramifying and intercommunicating air passages. A bird╒s lungs are small relative to its body volume--in a Mallard, about 2 percent compared with about 5 percent in humans--however, a bird╒s air sacs (structures not found in humans) are about 20 percent of its body volume and about 80 percent of the total volume of its respiratory system.Air SacsLike large, thin-walled bladders, air sacs extend from the bronchial system of the lungs to different parts of the bird╒s body. They may number six to fourteen in most birds, but eight to nine is typical. They have few blood vessels and play no direct part in oxygen and carbon dioxide exchange. Air coursing through them and the lungs provides more complete inner ventilation and helps regulate a bird's body heat. Thus, even without sweat glands, a bird is able to reduce its body heat by internal radiation and vaporization. Air sacs extend other branches, or ╥diverticula,╙ into nearly every part of the skeleton of some birds. The bones most frequently filled with air sac extensions are the humerus (upper ╥arm╙), femur (upper leg bone), ribs, and vertebrae. Most large birds such as albatrosses and eagles have extensive air sacs in long bones, which suggests that flight and light weight of bones are related. Air sacs give buoyancy to many water birds. In some diving birds such as loons, grebes, and anhingas, in which buoyancy would inhibit underwater swimming during the search for fish and other prey, air sacs within the bones are reduced or minimal.Air sacs just under the skin of the Northern Gannet and Brown Pelican seem to cushion the impact of these birds when they strike the water forcibly in their crash dives for fish. Others, such as Frigatebirds and male Prairie-chickens, during their courtship displays inflate their colorful neck bladders through the diverticula of the cervical air sacs. The male Prairie-chicken also uses inflated neck bladders as resonators for his booming courtship sounds.Sensory SystemsBirds have large brains compared to other vertebrates (aside from mammals)--generally ten times larger relative to body weight than any reptile relative. The olfactory lobes in bird brains are relatively small but the optic lobes are quite large. Within birds as a group, there are great variations in the size and proportions of various areas of the brain. Such differences are related to each bird╒s way of life. Corvids--jays, magpies, crows--seem to have the most highly developed brains.Bird sensory systems are well developed. Hearing is keen and second only to vision in importance. Olfaction, the sense of smell, is best developed in carrion eaters like vultures and ravens. While we do not yet clearly understand the role of smell in bird behavior, it is a subject of research. Taste is another sense which should be studied further in birds. The numbers of taste buds in birds compared to other vertebrates is quite low. Rabbits, for example, have 17,000 taste buds; most birds have fewer than one hundred.Touch is also variously developed. Some birds have specialized nerve endings found in no other animals. Many birds have touch sensors on their tongues and palates. The tongues of woodpeckers and the bills of ducks, flamingos, swallows, and parrots have other specialized nerve endings that detect touch or perhaps vibrations. The base of bird feathers are studded with nerve endings and birds notice the slightest touch on their feathers.Although all the senses work together to give a bird a cohesive view of the world, vision is the single most important sense, and it is instructive to examine it separately.Eyes and EyesightEyesAs a group, birds have the best vision among vertebrates. Birds have faster vision than humans; i.e., they are quicker in picking up details. With a single glance a bird immediately takes in a picture that a human might accumulate only by laboriously scanning the whole field of vision, piece by piece. Also, a bird╒s eyes have greater sensitivity for detecting movement. For example, movies work for human vision because we fuse the flickering images (usually projected at 24 frames per second) into a cohesive moving picture. A bird would see each frame separately, one after the other.Often the eyes of a bird weigh more than its brain. Large hawks and owls have eyes about equal in diameter to a human eye. In looking at a living bird, one sees a relatively small part of its eyes because the largest part is hidden by the lids and the skull. The eyes are partly protected and supported by the sclerotic ring, a series of thin bony plates. A thin, transparent membrane, called the nictitating membrane, lies under the bird's eyelids and is drawn across the eyeball to clean and moisten them, or to protect the eyes while the bird is in flight.How Birds Use Their EyesBirds╒ eyes are fixed in the sockets and generally they cannot ╥roll╙ them as a human can--turning the eyes about without moving the head. Birds turn their gaze in different directions by turning the head and neck. All birds have some degree of both monocular (one-eyed) vision--the ability to see independently with each eye--and binocular vision--the ability to see straight ahead using both eyes together. When a bird holds its head motionless, the area it sees varies greatly among different birds, depending on the placement of the eyes and angle of view. Pigeons, with their eyes on the sides of the head, have a total view of about 340 degrees; they can see almost everywhere around them except directly in back of the head. Their binocular vision is about 24 degrees of their 340-degree field of view. Hawks, with their eyes set more forward than those of pigeons, ducks, and most songbirds, have a smaller range of monocular (side) vision and cannot see behind them without turning the head. However, they have a wider range of binocular vision: about 35-50 degrees in front of them. The extent of monocular and binocular vision varies greatly in different birds. Owls, whose eyes are located toward the front of the head, have a total frontal field of about 60-70 degrees; however, owls can rotate their heads very rapidly about 270 degrees, which compensates for their less than adequate monocular vision. Visual AcuityThe resolving power (visual acuity) of a bird╒s eyes is based partly on the dense concentration of and high ratio of optic nerves to receptor cells (cones and rods). The latter, found in the retina at the back of a bird╒s eye, are minute visual cells which act as receptors to form the image that a bird sees. The cones (for their cone-like shape) operate in daylight or in bright light to enable a bird to distinguish colors and to receive sharp visual images. The rods (for their rod-like shape) function largely at night or at low light intensities, and are especially rich in the eyes of owls.EXTERNAL ANATOMYWhen we see a bird in the field, we apprehend it as an entire ╥gestalt╙--the total combination of all its visible characters, its sounds, and its behaviors (which birders refer to colloquially as ╥jizz╙). But to know a species truly, it is important to consider each part and what it contributes to the whole. External anatomy is of course what we see--feathers and coloration, beak shape, body size and shape, wings, legs, and feet.Every character has value in identifying a species. However, some are better than others in informing an observer about higher relationships (family and order). For example, bill shape is what biologists call an evolutionarily ╥plastic╙ character, meaning that bills are subject to relatively rapid evolutionary change. Consider the vast differences in bill shape of closely related birds such as the Darwin's finches or Hawaiian honeycreepers, birds whose bill shapes have diverged in different populations to exploit particular feeding niches. For information about higher relationships, it is better to rely on so-called conservative characters, ones that change slowly over time. Feet are the best conservative characters of external anatomy to use in inferring higher relationships.Feet and LegsMost birds, except loons and grebes, are remarkable in that they stand, not as a human does on the flat of the foot, but on their toes. The ╥instep╙ of a bird, instead of being short, as in a human, is an elongated bone (technically the ╥tarsometatarsus╙ but commonly shortened to ╥tarsus╙). Because it is long and visible, we may call it the bird╒s leg. As we look at a jay hopping over the ground, what appears to be its knee, bending backward instead of forward as in a human, is actually its heel. The bird╒s true knee joint, hidden from view by feathers, is the next joint above the bird╒s heel, and it bends forward when the bird squats, just as our human knee does. A bird's thigh (femur) is also obscured by feathers.Numbers of ToesMost birds have four toes. The common arrangement among songbirds (called ╥anisodactylous╙) is that the first toe (big toe, or ╥hallux╙) turns backward, and the other three turn forward; this permits the typical perching habit. The hallux in most birds is a functional toe that grows out backward at the same level as the others--a condition called ╥incumbent.╙ However, in many ground-dwelling birds, such as pheasants, the hallux is elevated and does not touch the ground. Adaptations of the Feet and Arrangement of the ToesThe feet of birds have become adapted over evolutionary time to serve birds╒ diverse ways of life. The following terms describe the arrangement of toes in various birds. Although a species is given as an example, in many cases these foot types are characteristic of an entire order, e.g., all pelicaniform birds are ╥totipalmate╙; all piciform birds are ╥syndactylous.╙ palmate (mallard): three front toes fully webbedtotipalmate (pelican): all four toes united by webssemipalmate (willet): three front toes joined by a web along only the basal half or less of their lengthlobate (grebe): toes bearing a fleshy lobelong-toed (jacana): very elongated toes modified for walking on water plantsperching (crow): toes modified for grabbing onto perchesraptorial (eagle): toes modified as muscular talonssyndactyl (kingfisher): two or more toes coalesced for a considerable part of their lengthpamprodactyl (swift): four toes turned forwardtridactyl (sanderling): only three toesdidactyl (ostrich): only two toesanisodactyl (robin): three toes forward, one back; the condition in all perching birdszygodactyl (woodpecker): yoke-toed two (II, III) in front; two (I, IV) behindheterodactyl (trogon): toes I and II turned backwards; III and IV forward
