Form And Function Of Bird

Body proportions

Birds arose as warm-blooded, arboreal, flying animals with forelimbs adapted for flight and hindlimbs for perching. This basic plan has become so modified, through the course of evolution, that in some forms it is difficult to recognize. The maximum size attainable by flying birds is limited by the fact that wing area varies as the square of linear proportions, and weight or volume as the cube. On the other hand, the minimum size is probably governed by another aspect of the surface-volume ratio: the relative increase, with decreasing size, in surface through which heat can be lost. The largest flying birds have highly pneumatic skeletons (part of the bone is replaced by air cavities) and other adaptations for reducing weight; the small size of some hummingbirds may be facilitated by the decrease in heat loss resulting from their becoming torpid at night.

When birds lose the power of flight, the limit on their maximum size is lifted, as can be seen in the ostrich and other ratite birds. Some birds (auks, diving petrels, and certain ducks) use the wings for propulsion underwater as well as in the air. When birds that fly underwater lose the ability to fly in air, the wings become highly modified as paddles, as in the penguins. The types of flight found in birds vary considerably. At least two major types of modifications for gliding or soaring are found. The albatrosses and some other seabirds have long, narrow wings and take advantage of winds over the oceans, whereas some vultures and hawks have broad wings with slotted tips and make more use of updrafts and winds deflected by hills. Short, broad wings are characteristic of chicken-like birds, which fly up with a rush of rapid wing beats. Birds like ducks, pigeons, and falcons, which fly rapidly with continuous wing beats, tend to have moderately long, pointed wings, while swifts and hummingbirds, with their narrow, curved wings fly rapidly and manoeuvre easily. The shape of a bird’s tail also appears to be related to flight. Forms with deeply forked tails, such as frigate birds, terns, and some swallows, manoeuvre easily, whereas the opposite extreme, long, graduated tails, are often found in rapid, direct fliers such as some parrots and doves. Woodpeckers and some other climbing birds have strong tail feathers with stout shafts, which they use as props while on the trunks of trees.

The bipedal gait, dictated by modification of the forelimbs for flight, necessitates manipulating food by the bill and feet and poses problems in balance. The relative lengths of the segments of the legs must be such that as the bird shifts from a standing to a sitting position, its centre of gravity remains over the feet. As some birds moved out of the trees and became terrestrial or aquatic, their legs were accordingly modified. The toes became shorter and the opposable first toe was lost in rapidly running forms like rheas and ostriches, and the toes became very long in birds that walk on aquatic vegetation or soft ground. In very large, slow-moving birds such as moas, the leg bones became very heavy. Wading birds developed long legs, and climbing birds developed short legs with strongly curved, sharp claws. In swimming and diving birds, webs developed beteen the toes or lobes on the sides of the toes.

Feathers and molt

Feathers are unique to birds and characteristic of them. Like the scales of reptiles, and those on the feet of birds, feathers are made of keratin, a fibrous protein also found in hair. Feathers vary considerably in structure and function. Contour feathers form most of the surface of the bird, streamlining it for flight and often waterproofing it. The basal portion may be downy and thus act as insulation. The major contour feathers of the wing (remiges) and tail (rectrices) and their coverts function in flight. Contour feathers grow in tracts (pterylae) separated by bare areas (apteria) and develop from follicles in the skin.

The typical contour feather consists of a tapered central shaft, the rachis, with paired branches (barbs) on each side. An unbranched basal section of the rachis is called the calamus, part of which lies beneath the skin. The barbs, in turn, have branches, the barbules. The barbules on the distal side of each barb have hooks (hamuli) that engage the barbules of the next barb. The barbs at the base of the vane are often plumaceous; i.e., lacking in hamuli and remaining free of each other. In many birds each contour feather on the body (but rarely on the wings) is provided with a complex branch, the aftershaft, or afterfeather, that arises at the base of the vane. The aftershaft has the appearance of a second, smaller feather, growing from the base of the first. Down feathers have loose-webbed barbs, all rising from the tip of a very short shaft. Their function is insulation, and they may be found in both pterylae and apteria in adult birds. They also constitute the first feather coat of most young birds. Filoplumes are hairlike feathers with a few soft barbs near the tip. They are associated with contour feathers and may be sensory or decorative in function. Bristle-like, vaneless feathers occur around the mouth, eyes, and nostrils of birds. They are especially conspicuous around the gape (corners of the mouth) of birds that catch insects in the air. Some bristles function as eyelashes on ground-dwelling birds, and the bristles over the nostrils may serve as filters.

The contour feathers are shed and replaced (molted) at least once a year, usually just after the breeding season. In addition, many birds have at least a partial molt before the breeding season. A typical series of molts and plumages would be juvenile plumage, postjuvenile (also called first prebasic) molt, first winter (or first basic) plumage, first prenuptial (or pre-alternate) molt, first nuptial (or alternate) plumage, first postnuptial (first annual, or second prebasic) molt, second winter (or basic) plumage, etc. Molt of the remiges and rectrices usually occurs as part of the annual molt and can be serial, from the innermost feather out (centrifugal), from the outermost in (centripetal), or simultaneous. Normally it is symmetrical between the right and left sides.

Colour in birds is caused by pigments or structure. Buffs, red browns, dark browns, and blacks are caused by melanins, pigments synthesized by the bird and laid down in granules. Yellows, oranges, and reds come from carotenoid or lipochrome pigments; these originate at least in part from the food and are diffused in the skin and feathers. Porphyrin feather pigments occur in birds but less frequently than melanins and carotenoids. Blue colours in feathers are structural, based on a thin, porous layer of keratin overlying melanin pigment. Most greens result from the addition of yellow pigment to the structural blue colour. Iridescent colours result from thinly laminated structure of the barbules and are enhanced by underlying melanin deposits. Birds’ feet are covered with scales like those of reptiles. The scales are occasionally shed, but the timing of this molt is not known. The toes are tipped with claws, and vestigial claws are not infrequently found on the tips of the first two digits of the wing.

The bill is covered with a sheet of keratin, the rhamphotheca, which in petrels and a few other birds is divided into plates. In birds that probe for food (kiwis, woodcock, etc.), many sensory pores are found near the tip of the bill. Both melanins and carotenoids are found in the rhamphotheca and in the scales of the feet. The skin of a bird is almost without glands. The important exception is the oil (uropygial) gland, which lies on the rump at the base of the tail. The secretion of this gland contains approximately one-half lipids (fats) and is probably important in dressing and waterproofing the plumage. In a few birds, the secretion has a strong, offensive odour. Some birds, in which the oil gland is small or absent, have a specialized type of feather (powder down) that grows continuously and breaks down into a fine powder, believed to be used in dressing the plumage.

Skeleton

The avian skeleton is notable for its strength and lightness, achieved by fusion of elements and by pneumatization (i.e., presence of air cavities). The skull represents an advance over that of reptiles in the relatively larger cranium with fusion of elements, made possible by the fact that birds have a fixed adult size. Birds differ from mammals in being able to move the upper mandible, relative to the cranium. When the mouth is opened, both lower and upper jaws move: the former by a simple, hingelike articulation with the quadrate bone at the base of the jaw, the latter through flexibility provided by a hinge between the frontal and nasal bones. As the lower jaw moves downward, the quadrate rocks forward on its articulation with the cranium, transferring this motion through the bones of the palate and the bony bar below the eye to the maxilla, the main bone of the upper jaw.

The number of vertebrae varies from 39 to 63, with remarkable variation (11 to 25) within the cervical (neck) series. The principal type of vertebral articulation is heterocoelous (saddle shaped). The three to 10 (usually five to eight) thoracic (chest) vertebrae each normally bear a pair of complete ribs consisting of a dorsal vertebral rib articulating with the vertebra and with the ventral sternal rib, which in turn articulates with the sternum (breastbone). Each vertebral rib bears a flat, backward-pointing spur, the uncinate process, characteristic of birds. The sternum, ribs, and their articulations form the structural basis for a bellows action, by which air is moved through the lungs. Posterior to the thoracic vertebrae is a series of 10 to 23 fused vertebrae, the synsacrum, to which the pelvic girdle is fused. Posterior to the synsacrum is a series of free caudal (tail) vertebrae and finally the pygostyle, which consists of several fused caudal vertebrae and supports the tail feathers. The sternum consists of a plate lying ventral to the thoracic cavity and a median keel extending ventrally from it. The plate and keel form the major area of attachment for the flight muscles. The bones of the pectoral girdle consist of the furcula (wishbone) and the paired coracoids and scapulas (shoulder blades). The sword-shaped scapula articulates with the coracoid and humerus (the bone of the upper “arm”) and lies just dorsal to the rib basket.

The coracoid articulates with the anterior (forward) edge of the sternum and with the scapula, humerus, and furcula. The furcula connects the shoulder joints with the anterior edge of the keel of the sternum. It consists of paired clavicles (collarbones) and, probably, the median, unpaired interclavicle. The bones of the forelimb are modified for flight with feathers. Major modifications include restricting the motion of the elbow and wrist joints to one plane, reduction of the number of digits, loss of functional claws, fusion of certain bones of the hand (the metacarpals and most of the carpals) into a carpometacarpus, and modification of the elements, especially those toward the tip of the limb (distal), for the attachment of feathers. The wing bones are hollow, and the cavity in the humerus, at least, is connected with the air-sac system. As a general rule, large flying birds have proportionally greater pneumaticity in the skeleton than small ones. The highly pneumatic bones of large flying birds are reinforced with bony struts at points of stress. The humerus, radius, and ulna are well developed. The secondary flight feathers are attached to the ulna, which thus directly transmits force from the flight muscles to these feathers and is therefore relatively heavier than the radius. Two small wrist bones are present: the radiale, or scapholunar, and the ulnare, or cuneiform. The former lies between the distal end of the radius and the proximal part (the part toward the body) of the carpometacarpus.

When the elbow joint is flexed (bent), the radius slides forward on the ulna and pushes the radiale against the carpometacarpus, which in turn flexes the wrist. Thus the two joints operate simultaneously. The U-shaped ulnare articulates with the ulna and the carpometacarpus. Anatomists differ on which bones of the reptilian hand are represented in the bird’s wing. Embryological evidence suggests that the digits are II, III, and IV, but it is possible that they are actually I, II, and III. The carpometacarpus consists of fused carpals (bones of the wrist) and metacarpals (bones of the palm), metacarpals II and III (or III and IV) contributing the greater part of the bone. The phalanges (bones of the fingers are reduced to one each on the outer and inner digits and two on the middle one. The primary flight feathers are attached to the carpometacarpus and digits, the number attached to each being characteristic of the various major groups of birds.

The pelvic girdle consists of three paired elements, the ilia, ischia, and pubes, which are fused into a single piece with the synsacrum. The ilium is the most dorsal element and the only one extending forward of the acetabulum (the socket of the leg). The ilium is fused with the synsacrum and the ischium, the latter of which is fused with the pubis. All three serve as attachments for leg muscles and contribute to the acetabulum, which forms the articulation for the femur. The leg skeleton consists of the femur (thighbone), tibiotarsus (main bone of the lower leg), fibula, tarsometatarsus (fused bones of the ankle and middle foot), and phalanges (toes). The fibula is largest at its proximal (upper) end, where it forms part of the knee joint and tapers to a point distally, never forming part of the ankle joint. The latter joint is simplified, there being but two bones involved: the tibiotarsus, consisting of the tibia (the so-called shinbone in man) fused with the three proximal tarsals (upper ankle bones), and the tarsometatarsus, resulting from the fusion of metatarsals I through IV and the distal row of tarsals. Metatarsals II through IV contribute most to the tarsometatarsus. The basic number of phalanges (sections) on the toes is two, three, four, and five, respectively; i.e., one more than the number of the toe. Most birds have four toes, the fifth being always absent, but there are many variations in the number of digits, or phalanges, representing reductions of the basic arrangement. The basic avian foot is adapted for perching. The first, or hind, toe (hallux) opposes the other three, and the tendons for the muscles that bend the toes pass behind the ankle joint in such a way that when the ankle is bent the toes are also. The weight of a crouched bird thus keeps the toes clasped around the perch.

Internal organs

The cardiac (heart) muscles and smooth muscles of the viscera of birds resemble those of reptiles and mammals. The smooth muscles in the skin include a series of minute feather muscles, usually a pair running from a feather follicle to each of the four surrounding follicles. Some of these muscles act to raise the feathers, others to depress them. The striated (striped) muscles that move the limbs are concentrated on the girdles and the proximal parts of the limbs. Two pairs of large muscles move the wings in flight: the pectoralis, which lowers the wing, and the supracoracoideus, which raises it. The latter lies in the angle between the keel and the plate of the sternum and along the coracoid. It achieves a pulley-like action by means of a tendon that passes through the canal at the junction of the coracoid, furcula, and scapula and attaches to the dorsal side of the head of the humerus. The pectoralis lies over the supracoracoideus and attaches directly to the head of the humerus. In most birds the supracoracoideus is much smaller than the pectoralis, weighing as little as one-twentieth as much; in the few groups that use a powered upstroke of the wings (penguins, auks, swifts, hummingbirds, and a few others), the supracoracoideus is relatively large. Avian striated muscles contain a respiratory pigment, myoglobin. There are relatively few myoglobin-containing cells inwhite meat,whereas “dark meat derives its characteristic colour from their presence. The former type of muscle is used in short, rapid bursts of activity, whereas the latter is characteristic of muscles used continuously for long periods and especially in muscles used during diving.

The circulatory system of birds is advanced over that of reptiles in several ways:

1. here is a complete separation between the pulmonary (lung) and systemic (body) circulations, as in the mammals;
2. the left systemic arch (aortic artery) is lost, blood passing from the heart to the dorsal aorta via the right arch;
3. the postcaval vein is directly connected with the renal portal that connects the kidneys with the liver; and
4. the portal circulation through the kidneys is greatly reduced. Birds’ hearts are large 0.2 to over 2.4 percent of body weight, as opposed to 0.24 to 0.79 percent in most mammals.

The avian lung differs from the type found in other land vertebrates, in containing fine tubes (capillaries) through which air passes and through the walls of which gas exchange takes place. Several pairs of nonvascular air sacs are connected with the lungs and extend into the pneumatic parts of the skeleton. The sound-producing organ in birds is the syrinx, located where the trachea (windpipe) divides into the bronchial tubes. The sounds are made by the flow of air setting up vibrations in membranes formed from part of the trachea, bronchi, or both. Muscles between the sternum and trachea or along the trachea and bronchi vary tension on the membranes.

The avian digestive system shows adaptations for a high metabolic rate and flight. Enlargements (collectively called the crop) of parts of the esophagus permit the temporary storage of food prior to digestion. The stomach is typically divided into a glandular proventriculus and a muscular gizzard, the latter lying near the centre of gravity of the bird and compensating for the lack of teeth and the generally weak jaw musculature. Otherwise, the digestive system does not vary markedly from the general vertebrate type. Like reptiles, birds possess a cloaca, a chamber that receives digestive and metabolic wastes and reproductive products. A dorsal outpocketing of the cloaca, the bursa of Fabricius, controls antibody-mediated immunity in young birds. The bursa regresses with age, and thus its presence or absence may be used to determine age.

The testes of the male bird are internal, like those of reptiles. Intromittent organs are found in only a few groups (waterfowl, cracids, tinamous, ratites). The distal part of the vas deferens (the seminal sac) becomes enlarged and convoluted in the breeding season and takes on both secretory and storage functions. In passerine birds, at least, this enlargement and the adjacent part of the cloaca form a cloacal protuberance, a swelling visible on the outside of the bird. Usually only the left ovary and oviduct are functional. The albumen, membranes, and shell are laid down in the oviduct as the egg moves down it. The gonads and accessory sexual organs of both sexes enlarge and regress seasonally. In the breeding season, the testes of finches may increase over 300-fold in volume over their winter size. Birds are homeothermic (warm-blooded) and maintain a body temperature of approximately 41° C (106° F). This temperature may be 1.7° C less during periods of sleep and up to 2° C higher at times of great activity. Feathers, including down, provide effective insulation. In addition, layers of subcutaneous fat add further insulation in penguins and in some other water birds. Reduction of heat loss from the feet in cold weather is accomplished by reducing blood flow to the feet and by a heat-exchange network in the blood vessels of the upper leg, so that the temperature of blood flowing into the unfeathered part of the leg is very low.

Birds differ from mammals in lacking sweat glands, hence heat loss is accomplished by rapid panting, which reaches 300 respirations per minute in domestic hens. Some heat dissipation can be accomplished by regulation of blood flow to the feet. In hot climates, overheating is often prevented or reduced by behavioural means by concentrating activities in the cooler parts of the day and seeking shade during the hot periods. Temporary hypothermia (lowered body temperature) and torpor are known for several species of nightjars, swifts, and hummingbirds. Torpor at night is believed to be widespread among hummingbirds. The heart rate of birds varies widely from 60 to 70 beats per minute in the ostrich to more than 1,000 in some hummingbirds. The kidneys lie in depressions that are located on the underside of the pelvis. The malpighian bodies, which are the active tubules of the kidney, are very small in comparison to those of mammals, ranging from 90 to 400 per cubic millimetre. More than 60 percent of the nitrogen is excreted as uric acid or its salts. There is some resorption of water from the urine in the cloaca, with uric acid remaining. There is no urinary bladder, the urine being voided with the feces. In marine birds, salt is excreted in a solution from glands lying above the eyes through ducts leading to the nasal cavity.

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