Birds, class Aves, are the only animals that have feathers. Aves is a large, highly diverse class with 8,600 species of water birds, songbirds, and birds of prey, including huge flightless runners, deep divers, and migratory species that travel thousands of miles each year. Although most birds have similar body structures, taxonomists, those who classify and name things such as animals and plants in groups, use variations in bone structure, muscles, and internal organs to distinguish one order from another. Variations in the shape and structure of the bills, wings, tails, and feet are also useful for classification and identification. Birds inhabit a wide variety of habitats and can be found on all the continents, most islands, and even on the open sea. They also have a wide range of size. The largest birds are the ostriches of Africa, up to 2m tall and weighing 136kg, and the great condors of the Americas, with wingspreads of up to 3m. The smallest known bird is Helena’s humming bird of Cuba, less than 6cm long and weighing less than 4g.
The skeleton system of birds is highly adapted for flight. In spite of its low weight, a bird’s skeleton is extremely strong. Hollow and fused bones are the two main bone types. Most bones` being hollow lessens overall body weight making flight more efficient. The skeleton of a bird is much lighter than any other kind of warm-blooded animal. However, this is achieved at a cost, and the skull of a bird is thin and susceptible to fracturing. Further lightening of the bird`s head has also been made possible by the absence of teeth. The other noticeable feature of a bird’s skull is the size of the eye sockets, to house the large eyes. This is probably because vision is a bird’s most crucial sense. The skeletal structure of the wing is relatively constant among the different species, but the actual shape of the feathers can be quite variable, depending on the flying capabilities of the species concerned. To provide effective thrust, the bones in the wing move together. As the bird’s wing beats downwards, special bones prevent its chest from being crushed. The thrust of the wing articulates with the scapula on each side of the body. The corocaid and the wishbone also provide reinforcement. The chest cavity is reinforced by the unusual structure of the ribs. The ribs have projections, known as incinerate process, which are directed backwards, overlapping each other. These help to provide support, especially in diving birds. In cross section, the wings are honeycombed. During flight, this lightens the load on the pectoral muscles which, attached to the keel of the sternum, may account for half of the birds body weight.
Flight is one of the most outstanding characteristics of birds. A bird’s body is adapted to meet the two major requirements for flight, which are low weight and high power. Unlike other vertebrates a bird has a skeleton that is very light in relation to body size, e.g. a pigeon’s skeleton accounts for only 4.4 per cent of its body weight. Birds have a compact, streamlined body, and the fusion of many of the bones gives the body the rigidity needed for flying. A birds wing bone is thin and hollow, with criss-cross reinforcements on the inside which provide support while adding little weight. Besides the unusual structure of bones, it is the presence of feathers that distinguishes birds from all other creatures that have conquered flight. Feathers are made of dead cells, and become dehydrated and are exceedingly light. Thus, they can be large and form the flight surface of the wings and tail without weighing much and without being susceptible to water or heat loss. Along with keeping a birds body warm, feathers function for flight. The long overlapping contour feathers streamline the body. These feathers form the wings and much of the tail. Wings made of contour feathers have much surface area and little weight. They are well adapted for fanning the air and for gliding on air currents.
The flight of birds is one of the most beautiful and wonderful things in nature. The make and arrangement of their feathers and bones, and their stream-lined shapes, from beak to tail, are instances of purposive adaptation. They soar with outstretched wings; they dart with folded wings. Their upward and downward mobility, as well as their stability in the air, and when they rest on their feet, have given many ideas to man in the science and art of aeronautics, But who taught or gave to birds this wonderful adaptation? None but God, Whose infinite Mercy provides for every creature just those conditions which are best adapted for its life. This is stated in a verse in the Qur’an (Mulk, 67.19):
Do they not observe the birds above them, spreading their wings and folding them in? None can uphold them except (God) Most Gracious: truly it is He that watches over all things.
Flying techniques depend on the principles of aerodynamics. Reducing air pressure and friction against the body requires perfect aerodynamic structure. Up to recent times, a rain drop was accepted as the most perfect aerodynamic shape. However, the shape of a bird’s body is now accepted as the most perfect, and aircraft are designed according to the aerodynamic structure of birds. Compared with birds, the latest aircraft are still clumsier and have less agility than birds. Moreover, hypoxia, which is a deficiency of oxygen and results in drowsiness, mental fugitive, headache, and sometimes euphoria, affects pilots severely and can cause them to lose their consciousness on flying upward. But this is not a hindrance for birds because the air between their feathers, their bodies and wings is absorbing and reducing air pressure, Along with this ability, there are some movements which birds can do easily, such as turning sharply, doing somersaults, steep ascents and descents, whereas manmade flying machines face some serious risks in attempting the same movements. Birds can also glide for long periods without beating their wings while aeroplanes cannot sustain flight with stopped engines. Vertical take off and landing is something many flying animals such as bees and birds have been able to do for millions of years. The structure of birds has inspired engineers to design specific modifications in recent models of aircraft, such as the F-14, F-18, and M1G-25, It is true that producing an aircraft requires complex skills and knowledge, improved steadily over a long time. But is it not marvellous that birds have been demonstrating perfect flight for millions of years?
Birds have become adapted to a variety of environments, and various species have very different types of beaks, feet, wings, tails, and behavioural patterns. Although all birds must eat frequently (because they do not store much and yet must maintain a very high metabolic rate), their choice of food varies widely among species-seeds, fruits, worms, molluscs, rodents, rabbits, fish, snakes, lizards, even dead animals. The shape of the bills reflects the birds feeding habits. For example, nectar-feeders have narrow, pointed bills; finches, which consume large quantities of seed, have short, stout bills; and birds that eat insects and berries have a non-specific bill shape and so on.
Birds also lack teeth and this probably reduces body weight for flight. Instead of teeth, their mouths are modified into bills. In addition to this modification, birds have another adaptation for digesting food. A bird swallows small bits of gravel, which act as teeth in the gizzard, mechanically breaking down food. An interesting feature of the bird’s digestive system is the crop, an expanded, sack-like portion of the digestive tract below the oesophagus, in which food is temporarily stored, and where food such as hard seeds is softened with mucus. As a storage organ, the crop allows the bird to feed rapidly and to store large amounts of food. The bird also produces a white secretion, sometimes called crop milk’, which is rich in proteins and fats, and birds use this secretion to feed their young. Birds (except for the ostrich) do not possess a bladder, and so the uric acid passes directly, through the uretors, to the cloaca. After reabsorbing water back into the bird’s body, uric acid is excreted in the form of a semi-solid, whitish concentrate.
Birds often build up high concentrations in their bodies because they consume salty foods or sea water and lose water through evaporation in the urine and feces. To rid themselves of this excess salt, these animals have salt glands, special secretary organs near the eye or in the tongue that remove excess salt from the blood and secrete it in a solution of tear-like drops. Sensors in the wall of a bird’s heart apparently monitor the osmotic pressure of the blood and send nerve signals to the brain that trigger activation of the salt glands. Thus, the animal can drink sea water and yet, by excreting some of the water and all the salts, achieve a net water gain. This is an adaptive mechanism that helps to maintain a light body weight.
Birds have a well-developed nervous system with a brain. They
rely heavily on vision; their eyes are proportionately larger than those of other vertebrates. Hearing is also well developed, in striking contrast to most animals, birds have developed the voice. The spoken word in human speech is different from the means of communication which birds have between each other. But no man can doubt that they have means of communication with each other, if he only observes the orderly flight of migratory birds which live in communities. This is also stated in the sacred book, Qur’an (al-Naml. 27.16):
And Solomon was David’s heir. He said: ‘O ye people! We have been taught the speech of Birds, and on us has been bestowed (a little) of all things.’ this is indeed Grace manifest (from God.)
Most birds have short simple calls that signal danger, or that influence feeding, flocking, or interaction between parent and young. Songs are usually more complex than calls and are performed mainly by males; they are related to reproduction, attracting and keeping a mate, claiming and defending territory.
Birds have a special system of air sacs that make fresh air almost continuously available to the sites of gas exchange. This provides the animal with the large quantities of oxygen needed for long distance, high- altitude flights -- even a flight over Mt. Everest. A bird’s many air sacs also lower the body weight relative to its size. Air sacs lying between certain flight muscles are squeezed and relaxed on each stroke of the wing and this helps increase the amount of air during inhalation and exhalation. The faster the bird flies, the more rapid is the circulation of air through the lungs.
The paired lungs are relatively small and nine or more hollow air sacs are attached to the lungs and fill much of the body cavity. They are like balloons that lighten the body and serve as reservoirs for air that will later be needed. Experiments show that air flows continuously during both inhalation and exhalation, in one direction through the lungs, and the air is renewed during each inspiration. Therefore, oxygen and carbon dioxide exchange occurs in the lungs’ air capillaries during both inhalation and exhalation. The direction of blood flow in the lungs is opposite to the that of air flow through the parabrocni, tiny, thin-walled ducts in the lungs. This counter current flow increases the amount of oxygen that enters the blood.
This system enables birds to do the intense work of flying, even at high altitudes, and still maintain a high body temperature. Interestingly enough, rapid breathing also contributes to such a high performance; a Venezuela hummingbird, the sparkling violet ear, breathes 330 times per minute at sea level and 380 times per minute at high altitudes. This is a very high heart beat such as no other warm-blooded animal can accomplish.
The very effective respiratory and circulatory systems provide enough oxygen to the cells to permit a high metabolic rate. This is another important adaptation of birds: the maintenance of a high and constant body temperature in spite of changes in the external environment. High metabolic rate is necessary for the tremendous muscular activity required for flying. Some of the heat generated by metabolic activities is used to maintain a constant body temperature (between 35 and 42 degrees C) which permits birds to remain active in cold climates. Birds are among very few animals able to maintain a constant body temperature. Though birds are sometimes called warm-blooded, the preferred term is homeothermic.
One of the most interesting types of behaviour in birds is migration. Migration is the instinctive movement of animals, usually between their wintering grounds and their breeding grounds. Migration provides birds a better chance of surviving. By instinct, migrating birds make their long journeys each year to reach more favourable habitats. Some birds, such as the golden plover and arctic tern, fly from Alaska to Patagonia, South America, and back each year, flying 25,000km en route.
Migration is probably set in by several external stimuli, such as temperature and length of daylight. It is also triggered by the secretion of hormones. For many species the direction of migration is probably based on such factors as the bird’s observation of the sun and stars and even by the earth’s magnetic field. Few modern biological discoveries have been more surprising than the finding, in the 1970s, that bird’s, bacteria, and perhaps many other types of organisms orient their bodies to the earth’s magnetic field, How such animals can have acquired this magnificent ability is a question yet to be answered. How can living things perceive a force as elusive as magnetism?
Many birds and at least one mammal-the porpoise-have a small number of tiny crystals of magnetite (a form of iron oxide) in tissues near the brain. Likewise, magnetobacteria, have magnetite crystals in their cytoplasm and swim toward or away from the poles of an applied magnetic field. How some higher organisms use magnetic particles to sense the direction of the lines of magnetic force generated by the earth’s core remains a subject of research. One hypothesis suggests that the crystals in, for example, a pigeons head, function like tiny compass needless, and mechanoreceptors might detect their relative movement. Other work indicates that in the rat, changes in magnetic yield can alter enzyme activity and levels of melatonin in the pineal gland. Still other work suggests that regularly arrayed layers (also present in pineal glands) undergo certain changes as the head and eyes move relative to the earth’s magnetic field. They could allow the animal to sense magnetic fields, but not by a mechanoreceptor mechanism. Whatever the mechanisms, homing, migration and many other directional animal behaviours depend on detecting magnetic fields.
Birds with their perfect features are wonders in nature to be marvelled at as such. All creation, both animate and inanimate, celebrates Gods praise and bears witness to His power, wisdom and goodness. We can aspire to true knowledge of God through observing the wonders in nature. However, this is possible only if people do not insist on their positivistic viewpoint and see the direct manifestations of God’s Names in His creation:
The seven heavens and the earth, and all beings therein, declare His glory: There is not a thing but celebrates His praise. And yet you understand not how they declare His glory! Truly, He is Off-Forbearing,’ Most Forgiving! (Bani Israll. 17.44)