PoultryPosted on May 18, 2018 - Last modified: September 8, 2018
The birds are flying animals which are characterized mainly by being the only species to have feathers. A more elaborate definition would indicate that they are warm-blooded vertebrates more related to reptiles than mammals and that they have a four-chambered heart (just like mammals), wing-modified forelimbs (a trait shared with bats), a hard-shelled egg, and keen vision, the main sense on which they rely for information about the environment. His sense of smell is not highly developed and his hearing range is limited. Most of the birds are diurnal. More than 1.000 extinct species have been identified from fossil remains.
Table of Contents
Birds emerged as warm-blooded, arboreal, flying creatures, with front legs adapted for flight and hind legs for perching. This basic plan has been modified so much in the course of evolution that in some ways it is difficult to recognize.
Among the flying birds, the wandering albatross has the largest wingspan, up to 3,5 meters, and the trumpeter swan perhaps the largest, 17 kg. In larger flying birds, part of the bone is replaced by air cavities (pneumatic skeletons) because the maximum size achievable by flying birds is limited by the fact that the wing area varies as the square of linear proportions, and the weight or volume as the cube. During the Pleistocene (2,6 million to 11.700 years ago) a bird called Teratornis incredibilis lived. Although similar to today's condors, it had an estimated wingspan greater than about 5 meters and was by far the largest flying bird known.
The smallest living bird is generally recognized as the Cuban bee hummingbird, measuring 6.3 cm and weighing less than 3 grams. The minimum size is probably governed by another aspect of the surface-volume ratio: the relative increase, with decreasing size, in the surface through which heat can be lost. The small size of some hummingbirds can be facilitated by a decrease in heat loss as a result of their clumsiness at night.
When the birds lose the power of flight, the limit of their maximum size increases, as can be seen in the ostrich and other ratids such as the emu, the cassowary and the rhea. The ostrich is the largest living bird and can measure 2,75 meters and weigh 150 kg. Some recently extinct birds were even larger: New Zealand's largest moas and Madagascar's elephant birds may have reached over 3 feet.
The ability to fly has allowed for almost limitless diversification of birds, so that they are now found virtually everywhere on Earth, from occasional stragglers over the polar caps to complex communities in tropical forests.
In general, the number of species that are found breeding in a given area is directly proportional to the size of the area and the diversity of available habitats. The total number of birds is also related to factors such as the position of the area with respect to migration routes and wintering grounds for species that nest outside the area.
Due to their body structure and feather coverage, birds are the best fliers among animals, better than insects and bats. There are, however, considerable differences in capacity between various birds. Penguins cannot fly, instead spending much of their time in the water swimming with their paddle-like wings. Birds like ostriches and emus have rudimentary wings but are permanently upright. At the other extreme, long-winged swifts and frigates move from their perches only to fly, never to walk. Most birds alternate walking or swimming with their flight.
Birds generally fly when they have a considerable distance to travel; however, there are exceptions. The California mountain quail makes its annual migrations on foot through the mountains. Guillemots off the coast of Greenland migrate south by swimming; They begin their journey before the young have grown their flight feathers and before some of the adults have at least regrowth their newly molts. Adélie penguins can ride north on drifting icebergs; As the nesting season approaches, they swim back to the Antarctic continent and then walk on the ice to their breeding grounds many kilometers inland.
Birds fly by flapping their wings, targeting mainly with their tails. Compared to the parts of an airplane, the wing of a bird acts as a wing and a propeller. The basal part of the wing supplies most of the bearing surface, the wingtip most of the propelling force. The wing of a bird has many adjustable characteristics: it can be shortened or lengthened by bending; tip feathers can be extended or closed; the angle of the entire wing or of its parts - on one side or both - can be altered. All these adjustments make the aerodynamics of a bird's wing much more complicated than that of an airplane; consequently, the flight of a bird is much more varied and adaptable.
The types of flight found in birds vary considerably, and different types of wings correlate with different types of flight. At least two main types of modifications are found for gliding or flying. Albatrosses and some other seabirds have long, narrow wings and take advantage of the winds over the oceans, while some vultures and hawks have wide wings with slotted tips that allow greater use of updrafts and winds blown off hills.
The shape of a bird's tail also appears to be related to flight. The forked tails of frigates and terns allow for rapid changes of direction, and the barn swallow uses its deeply forked tail to make the intricate patterns of its graceful flight. A goshawk chasing its prey through the forest uses its long tail to make quick turns. However, there is such diversity in the tails of birds that precise size and shape are probably not of critical importance.
The speed with which birds fly also varies greatly, and of course each of them can vary their speed. Data on the speed of flight of birds is difficult to assess. One of the complicating factors is that the bird's speed relative to the ground can depend on the strength of the wind.
Some birds have completely lost the power of flight during the course of evolution. However, the close similarity in the basic structure of flightless and flying birds indicates that they all had a common flying ancestor. The rudimentary wings and flightless status of penguins and ratites (ostriches and the like) are therefore a secondary and specialized condition. That lack of flight is a secondary condition is even more evident in other flightless birds that belong to families in which most of their members are capable of flight.
Loss of flight appears to occur more frequently on isolated islands where there are no mammalian predators. In New Zealand, where there are no native land mammals of any kind, there were many species of extinct flightless moas, and there are still kiwis, penguins, and flightless rails, as well as a duck, owl, and various barely-flying songbirds. Ratids from South America (rhea), Africa (ostrich) and Australia (cassowary) present an apparent contradiction with this correlation of mammal-free island habitats with the lack of flight of birds. However, another adaptation, their large size, has allowed these birds to escape mammalian predators.
Walk and jump
The bipedal gait of birds, dictated by the modification of the front legs for flight, requires manipulation of food by the beak and legs. This raises balance issues. The relative lengths of the leg segments should be such that, as the bird changes from standing to sitting, its center of gravity remains over the legs. As some birds moved away from the trees and became terrestrial or aquatic, their legs were modified accordingly. In very large, slow-moving birds, such as moas, the leg bones became very heavy. The toes became shorter, and the opposing first toe has been lost in fast-running forms such as rheas and ostriches. The ostrich is the fastest runner, crossing sections of savanna at a speed of 72,5 km / h. The toes became very long in birds that walk on aquatic vegetation or soft soils. Jacanas, with their very elongated fingers and claws, walk on floating aquatic weeds, and herons, with their long legs, wade in shallow water. Wading birds developed long, thin legs, and climbing birds developed short legs with sharp, strongly curved claws. In swimming and diving birds, cobwebs develop between the fingers or lobes on the sides of the fingers.
Land birds like pheasants tend to walk; arboreal songbirds tend to leap as they travel from branch to branch. Tree dwellers, such as woodpeckers, toucans, and other members of the Piciformes order, as well as parrots, can easily climb up and down trees because their outer two toes face backward; in almost all other birds, only one finger looks back. Parrots often walk along branches, and house sparrows jump when they hit the ground, while palm warblers walk on the ground and some songbirds, such as American robins and European blackbirds, can both walk and skip. Some birds with small legs, such as swifts, hummingbirds, bee-eaters, and many hornbills, use their legs only for perching and rarely walk. Other birds with sturdy legs, such as guinea fowls and rails, move most of the time on foot.
The usual position of a bird's body when walking is more or less parallel to the ground. But penguins, with their paws far from the back of their bodies, stand up as they walk. When the Adélie penguin makes its long walk on snow-covered ice to its breeding grounds, it can vary its awkward gait with periods of tobogganing, that is, sliding on its chest and propelling itself with the thrust of its feet.
Swim and diving
Some birds use their wings for propulsion underwater and in the air. Penguins' wings have been greatly modified to become blades that allow them to "fly" underwater; they only use their webbed feet to rule. Several other waterfowl have become so adapted to swimming that they are practically defenseless on land. In this class are the madmen, who clumsily shuffle the few feet from the water to their nests. Swimming in birds is usually correlated with webbed feet, but coots and grebes, which only have lobes on their toes, also swim and dive, and galpules, which do not have cobwebs or lobes, usually swim. On the other hand, frigates, with partially webbed feet, never swim.
Some birds, such as the mallard, generally swim at the surface, feeding only as far as they can go by submerging the head. Other ducks commonly dive to the bottom in search of food, and cormorants and grebes chase fish underwater. Loons are sometimes caught at remarkable depths in fishermen's nets and on cast lines, indicating that they can dive to depths of up to 61 meters. Emperor penguins, however, are the best divers, having been recorded at depths of 483 meters.
Birds are highly dependent on innate behavior, automatically responding to specific visual or auditory stimuli. Even much of their feeding and reproductive behavior is stereotyped. Care of the feathers is vital to keeping the wings and tail in flight condition and the rest of the feathers in place, where they can act as insulation. Therefore, the grooming, greasing, shaking and stretching movements are well developed and used regularly.
Some movements, such as the simultaneous stretching of a wing, a leg, and half of the tail (all on the same side) are generalized, if not universal, among birds. Stretching both wings up, whether bent or extended, is another common movement, as is shaking the entire body starting at the rear end. Other movements have evolved in relation to bathing, either in the water or in the dust. These comfort movements have often been ritualized as components of displays.
Many birds maintain a minimum distance between themselves and their neighbors, as can be seen in the spacing of a flock of swallows perched on a wire. In the breeding season, most species maintain territories, defending areas ranging from the immediate vicinity of the nest to large areas where the pair not only nest but also feed. The frequency of actual fighting is greatly reduced by ritualized threats and displays of appeasement. The birds range from the solitary (e.g., many raptors) to the highly gregarious, such as the guanay cormorants of the Peruvian Current off the west coast of South America, which nest in huge colonies of hundreds of thousands and feed on large flocks of boobies and pelicans.
Auditory cues, like visual cues, are nearly universal among birds. The most familiar vocalization of birds is what is often called "song." It is a striking sound (not necessarily musical) that is used, especially at the beginning of the breeding season, to attract a mate, to warn of another bird of the same sex, or both. As such, it is often associated with the establishment and maintenance of territories.
The individual variations in the songs of many species are well known, and it is believed that some birds can recognize their partners and neighbors by this variation. Many other types of vocalizations are also known. Pairs or flocks can be held together by means of a series of soft placement notes. Alarm notes alert other individuals to the presence of danger; in fact, the American robin (and probably many other species) uses one note when it sees a hawk above its head and another when it sees a predator on the ground. Begging calls are important to encourage parents to feed their children. Other calls are associated with aggressive situations, courtship and mating. Non-vocal sounds are not uncommon. Some snipe and hummingbirds have narrow tail feathers that make loud sounds when the birds are flying, as do the narrow outer primaries of the American woodcock. The elaborate courtship of grouse includes vocalizations, as well as foot tapping and wing noises. Beak claps are a common part of courtship in storks, and beak clicks are a common threat from owls.
Most birds build nests in which they lay their eggs. Nests vary greatly: they can be a scratch in the sand, a deep burrow, a hole in a tree or rock, an open cup, a globular or replica-shaped mass with a side entry tube, or an elaborately hanging structure. woven.
The materials from which nests are made also vary widely. Some nests are lined with small stones, and others are built with earth or mud with or without plant material. Sticks, leaves, algae, roots and other plant fibers are used alone or in combination. Some birds look for materials of animal origin such as feathers, horsehair or snakeskin. Nest materials can be held together by weaving, sewing, or felting the materials themselves or with mud or cobwebs. Swifts use saliva to glue nest materials and to secure the nest to the supporting structure. In at least one species of swift, the entire nest is made of saliva and is the most precious ingredient in bird's nest soup in the East.
All birds incubate their eggs except megapods, which depend on heat generated by decaying vegetation or other external sources, and brood parasites such as cuckoos and cowboys, which lay their eggs in the nests of other species. Murres and king and emperor penguins do not build nests, but rather hatch with the egg resting on their feet.
The first birds were probably insectivorous, like many other modern birds, and the latter have developed many specializations in catching insects. Swifts, swallows, and nightjars have large spaces to trap insects on the wing; some woodpeckers can reach worms that bore into wood, while others can catch ants by burrowing in the ant hills with their long sticky tongues; whippers dig into the ground with their picks; creepers and woodpeckers probe the cracks in the bark; and warblers gather insects from many types of vegetation.
Birds of prey (raptors and owls) have developed claws and pecks to feed on larger animals, and vultures have bare heads and pecks to feed on carrion. Herons have spear beaks and trigger mechanisms on their necks to catch fish, while kingfishers, terns, and boobies dive into the water after similar prey. Some have long beaks to capture worms and other invertebrates. Of the many types of birds that feed on plant material, most use seeds, fruits or nectar, which have a high nutritional value; the leaves and buds are consumed by fewer species. While some types of birds feed entirely on a single type of food, others can consume a wide range of foods, and many have seasonal changes in diet.
Form and function
The structures associated with flight, even if they are vestiges or specialized for terrestrial or aquatic locomotion, easily distinguish birds from other animals. While various skeletal and internal features are diagnostic of birds, feathers are unique and present in all birds. Their sound-producing organ, the syrinx, is also unique to birds. This avian analog to the larynx is more developed in songbirds. The syrinx is located in the place where the trachea divides into the bronchi. Sound is produced by the flow of air that vibrates the membranes formed by part of the trachea, the bronchi, or both.
Like the scales of reptiles and those of the legs of birds, feathers are made of keratin, a fibrous protein that is also found in hair. Feathers vary considerably in structure and function. Contour feathers make up most of the bird's surface, making it more aerodynamic to fly and often waterproofing it. The basal portion can be soft and therefore act as an insulator. The main feathers around the wing (remiges) and tail (rectrices) and their coverts function in flight. Contour feathers grow in tracts (pteriaceae) separated by bare areas (apteria) and develop from skin follicles.
The typical contour feather consists of a conical central axis, the rachis, with paired branches (barbs) on either side. An unbranched basal section of the spine is called a calamity, part of which is under the skin. The beards, in turn, have branches, the beards. The chins on the distal side of each beard have hooks (hamuli) that hook the chins of the next beard. The barbs at the base of the fin are often feathery, that is, they lack hamuli and remain free from each other. In many birds each contour feather on the body (but rarely on the wings) is provided with a complex branch, the stern shaft, or rear feather, that arises at the base of the fin. The posterior axis has the appearance of a second, smaller feather that grows from the base of the first.
Down feathers have loose fabric barbs, which rise from the tip of a very short shaft. Their function is isolation, and they can be found in both pteriaceae and apteria in adult birds. They also make up the first layer of feathers for most young birds. Philoplumes are hairy feathers with a few soft barbs near the tip. They are associated with contour feathers and can be sensory or decorative depending on the function. Viscera-like feathers, similar to bristles, occur around the mouth, eyes and nostrils of birds. They are especially visible around the opening (corners of the mouth) of birds that catch insects in the air. Some bristles function as eyelashes in birds that live on the ground, and the bristles above the nostrils can serve as filters.
The skeleton of birds is distinguished by its resistance and lightness, achieved by fusion of elements and by pneumatization (presence of air cavities).
The skull represents an advance over that of reptiles in the relatively larger skull with fusion of elements, which is made possible by the fact that birds have a fixed adult size. Birds differ from mammals in that they can move the upper jaw rather than the lower jaw, relative to the skull.
When the mouth is opened, both the lower and upper jaws move: the former by a simple hinge-shaped joint with the bony square at the base of the jaw, the latter through the flexibility provided by a hinge between the jaws. frontal and nasal bones. As the lower jaw moves downward, the square swings forward at its joint with the skull, transferring this motion through the palate bones and the bony bar below the eye to the maxilla, the main bone of the jaw. higher.
The number of vertebrae varies from 39 to 63, with a notable variation (11 to 25) within the cervical series. The main type of vertebral joint is heterocellular (saddle-shaped). Each of the pectoral (thoracic) vertebrae 3 to 10 (usually 5 to 8) normally carries a pair of complete ribs consisting of a dorsal vertebral rib that articulates with the vertebra and with the ventral sternal rib, which in turn once articulates with the sternum. Each vertebral rib has a flat back spur, the uninate process, characteristic of birds.
The sternum, ribs, and their joints form the structural basis for the action of a bellows, by which air moves through the lungs. Posterior to the thoracic vertebrae there is a series of 10 to 23 fused vertebrae, the synsacrum, to which the pelvic girdle is fused. Posterior to the synsacrum there 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 that lies ventral to the thoracic cavity and a median keel that extends ventrally from it. The plate and keel form the main attachment area for the flight muscles. The bones of the pectoral girdle consist of the wishbone (furcula) and the paired coracoids and shoulder blades (scapulae). The sword-shaped scapula articulates with the coracoid and upper "arm" (humerus) and lies just dorsal to the basket of ribs. The coracoid articulates with the anterior border of the sternum and with the scapula, humerus, and furcula. The furcula connects the shoulder joints with the anterior border of the keel of the sternum. It consists of paired clavicles (clavicles) and probably the median, unpaired interclavicle.
The bones of the forelimb are modified to fly with feathers. Major modifications include restriction of movement of the elbow and wrist joints to one plane, reduction in the number of digits, loss of functional claws, fusion of certain bones of the 'hand' (the metacarpals and most of the carpals) in a carpometacarpus, and the modification of the elements, especially those towards the tip of the limb (distal), for the fixation of feathers.
The wing bones are hollow, and the socket in the humerus is connected to the air-sac system. As a general rule, large flying birds have proportionally greater pneumatics on the skeleton than small ones. The highly pneumatic bones of large flying birds are reinforced with bony struts at stress points.
The humerus, radius, and ulna are well developed. The secondary flight feathers are attached to the ulna, which transmits the force of the flight muscles directly to these feathers and is therefore relatively heavier than the radius. Two small bones of the wrist are present: the radial, or scapholunar, and the ulna, or cuneiform. The first is located between the distal end of the radius and the proximal part (the part towards the body) of the carpometacarpus. When the elbow joint flexes (bends), the radius slides forward into the ulna and pushes the radial against the carpometacarpus, which in turn flexes the wrist. In this way, the two joints work simultaneously. The U-shaped ulna articulates with the ulna and carpometacarpus. Anatomists differ in which bones of the reptilian "hand" are represented on the wing of the bird.
The pelvis consists of three paired elements, the ilium, the ischemia, and the pubic hair, which are fused in one piece with the synsacre. The ilium is the most dorsal element and the only one that extends forward from the socket of the leg (acetabulum).
The ilium fuses with the synsacrum and ischium, the latter of which fuses with the pubis. All three serve as accessories for the leg muscles and contribute to the acetabulum, which forms the joint of the femur. The skeleton of the leg is made up of the femur, the main bone of the lower leg (tibiotarsus), the fibula, the joined bones of the ankle and middle foot (tarsometatarsus), and the toes (phalanges).
The fibula is largest at its upper end, where it forms part of the knee joint and tapers to a distal point, never forming part of the ankle joint. This last joint is simplified, since there are only two bones involved: the tibiotarsus, formed by the tibia (the so-called shin bone in man) fused with the three upper bones of the ankle (proximal tarsals), and the tarsometatarsus and the distal row of tarsals. The metatarsals II through IV are the major contributors to the tarsometatarsus.
Most birds have four toes, the fifth is always absent, but there are many variations in the number of digits, or phalanges, representing reductions from the basic arrangement.
The basic bird's foot is adapted for perching. The first toe, or back toe, of the foot (hallux) opposes the other three, and the tendons of the muscles that bend them pass behind the ankle joint in such a way that when the ankle is bent the toes also bend. make. The weight of a crouched bird keeps the toes around the perch.
Muscles and organs
The cardiac (heart) muscles and the smooth muscles of the viscera of birds resemble those of reptiles and mammals. The smooth muscles of the skin include a series of tiny feather muscles, usually a pair ranging from one feather follicle to each of the four surrounding follicles.
Some of these muscles act to lift the feathers, others to press them down. The striated (striped) muscles that move the limbs are concentrated in the girdles and 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 is found at the angle between the keel and the sternum plate 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 pectoral lies on the supracoracoideus and attaches directly to the head of the humerus. In most birds the supracoracoideus is much smaller than the pectoralis, weighing only one twentieth of its weight; In the few groups that use a powered wing stroke (penguins, elk, swifts, hummingbirds, and a few others), the supracoracoideus is relatively large.
The striated muscles of birds contain a respiratory pigment, myoglobin. There are relatively few cells that contain myoglobin in "white meat", whereas "dark meat" derives its characteristic color from its presence. The first type of muscle is used in short, fast bursts of activity, while the second is characteristic of muscles used continuously for long periods and especially muscles used during diving.
The circulatory system of birds is advanced over that of reptiles in several ways: (1) there is a complete separation between the pulmonary circulation (lungs) and the systemic circulation (body), as in mammals, (2) the systemic arch left (aortic artery) is lost, blood passing from the heart to the dorsal aorta through the right arch, (3) the postcaval vein is directly connected to the renal portal connecting the kidneys to the liver, and (4) the Portal circulation through the kidneys is greatly reduced. The hearts of birds are large: 0,2 to more than 2,4 percent of body weight, compared to 0,24 to 0,79 percent in most mammals.
The lung of birds differs from the type found in other land vertebrates in that several pairs of non-vascular air sacs are connected to the lungs. These extend to the pneumatic parts of the skeleton. The muscles between the sternum and the trachea or along the trachea and bronchi vary the tension on the membranes.
The digestive system of birds shows adaptations for a high metabolic rate and flight. The enlargements of the esophagus, collectively called the culture, allow for temporary storage of food before digestion. The stomach is typically divided into a glandular proventriculus and a muscular gizzard, the latter located near the center of gravity of the bird and compensating for the lack of teeth and the generally weak musculature of the jaw. Otherwise, the digestive system does not vary markedly from the general type of vertebrates.
Like reptiles, birds have a cloaca, a chamber that receives digestive and metabolic waste and reproductive products. A dorsal pocket of the cloaca, Fabrizio's bursa, controls antibody-mediated immunity in young birds. The bursa regresses with age, and therefore its presence or absence can be used to determine age.
The male's testicles are internal, like those of reptiles. The intestinal organs are found in few groups (waterfowl, curasows, tinamous, ratids). The distal part of the vas deferens (the seminal sac) is enlarged and entangled during the breeding season and assumes secretion and storage functions. In songbirds this enlargement and the adjacent part of the cloaca form a cloacal bulge, a visible swelling on the outside of the bird. Usually only the left ovary and oviduct are functional.
As the egg travels down the oviduct, the albumin, membranes, and shell are deposited. The gonads and accessory sex organs of both sexes are seasonally enlarged and regressed. In the breeding season, finch testes can increase more than 300 times their winter size.
Birds are homeothermic (warm-blooded) and maintain a body temperature of approximately 41 ° C. This temperature may be slightly lower during periods of sleep and slightly higher during vigorous activity. Feathers, including down, provide effective insulation. Additionally, the layers of subcutaneous fat add more insulation to penguins and some other waterfowl. Heat loss through the feet in cold weather is minimized 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 the blood flowing into the non-feathered part of the leg is very low.
Birds do not have sweat glands. Excess heat is dissipated by rapid panting, which reaches 300 breaths per minute in domestic chickens. Some heat can also be lost by regulating blood flow to the feet. In hot climates, overheating is often prevented or reduced through behavior by concentrating activities in the colder parts of the day and seeking shade during hot periods. Temporary hypothermia (decreased body temperature) and lethargy are known in several species of nightjar, swifts, and hummingbirds. Nocturnal torpor 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 1000 in some hummingbirds.
The kidneys are in depressions that are located in the lower part of the pelvis. Malignant bodies, which are the active tubules of the kidney, are very small compared to those of mammals, ranging from 90 to 400 per cubic millimeter. More than 60 percent of the residual nitrogen is excreted as uric acid or its salts. There is some reabsorption of water from the urine into the cloaca, leaving uric acid. There is no urinary bladder, urine is emptied with feces. In seabirds, salt is excreted in a solution from the glands above the eyes through tubes that lead to the nasal cavity.