AMAZING FLYING MACHINES
Birds’ flight have many secrets which are yet to be unraveled. They can fly up and land without any preparatory run whereas man-made flying machines need long strips to do this. Even today man has not been able to understand how birds withstand the tremendous strain of their flights. Sometimes they have to fly over sea, deserts and inhospitable mountains, where they have no chance of taking rest or “re-fuelling” – having something to eat – but still they survive.
Take birds’ maneuverability, speed, take-off, landing or lifting capacity they are incomparable, except for insects. Man or his machines stand nowhere; birds can carry much greater loads (relative to their weight and size, of course) than the most powerful of airplanes. If we take the speed, a modern aircraft flies at an average speed of 1,000 km an hour. A homing pigeon can only make 60 km and hour, but if we compare the size of both, the airplane is greatly outraced by the pigeon. Within the same stretch of time the plane will fly a distance equal to 1,500 lengths of its body, the pigeon 5,000 of its body length.
For flying, birds need a lighter and powerful body. Their bones weigh very little, for they are mostly thin and hollow, but struts inside make them strong. Big, powerful muscles joined to the wings and breastbone flap the wings up and down. Muscles get energy from oxygen and sugar. These are pumped in the blood to the muscles by a big, fast-beating heart. The oxygen comes from the breathed-in-air. When bird breathes air into its lungs, some air is stored in other hollows in the body. As a bird breathes out, this extra air flows through the lungs. So some oxygen is always entering a bird’s blood as it flies. This keeps its muscles working.
Birds capable of rapid and sustained flight have long, narrow wings, e. g. the common swift, hobby, terns etc. These birds have a slender body that offers minimum resistance to the air.
Feathers are among the most complicated integumentary appendages found on the bodies of vertebrates and are formed in tiny follicles in the epidermis, or outer skin layer, that produces proteins named keratin.
They are of two basic types: (1) vaned feathers — they cover the outer surface of the body, and (2) down feathers — that are found underneath the vaned feathers. The pennaceous or contour feathers are in fact vaned feathers that arise from tracts and cover the whole body. A third type is hair-like filoplumes, which are rare type of feathers and (if present in a bird) grow along the fluffy down feathers. In some perching birds, filoplumes arise uncovered beyond the contour feathers on the neck. The flight feathers of wings also called remiges, and the flight feathers of tail, the rectrices, are the most important feathers for flight. A normal vaned feather features a main shaft, called the rachis. Merged to the rachis are a chain of branches, or barbs, which themselves are branched to form the barbules. These structures (barbules) have tiny hooks called barbicels for cross-attachment.
Usual characteristic of down feathers is that they are fluffy because they do not have barbicels, so their barbules float free of each other, allowing the feather to trap much air and provide excellent thermal insulation. Hatchlings of some species have an unusual kind of natal down (neossoptiles) that is pushed out when the normal feathers (teleoptiles) emerge.
Flight feathers become rigid so as to work against the air in the down-stroke but yield in other directions. It is noted that the model of orientation of β-keratin fibers in the feathers of flying birds are different from the flightless birds. The fibers are better lined up in the centre of the feather and less lined up towards the tips.
Functions of Feathers
Feathers perform many functions — Although bird feathers are quite light weight, they still weigh two to three times more than the bird’s skeleton, since many bones are hollow and contain air sacs. Feathers insulate birds from water and cold. Individual feathers in the wings and tail play important roles in controlling flight. Colour patterns of plumage serve as camouflage against predators, and also help predatory birds, looking for a meal, to approach their prey unnoticed. Differences in colours and feather patterns are part of sexual dimorphism in many species and are mainly important in selection of mating pairs. In some cases there are differences in the UV reflectivity of feathers across sexes even though no differences in colour are noted in the visible range. In male Club-winged Manakins (Machaeropterus deliciosus), wing feathers have special structures that are used to produce sounds by stridulation.
There are certain birds that have a supply of powder down feathers that grow continuously, with small particles breaking off on a regular basis from the ends of the barbules. These particles produce a powder that filters through the feathers on the bird’s body and acts as a waterproofing agent and a feather conditioner.
Grebes, freshwater diving birds, some of which visit the sea while migrating in winter, are strange in their habit of swallowing their own feathers and also feeding them to their young. Observations on the diet and feather eating frequency suggest that eating feathers particularly the down from their flanks helps in forming easily ejectable pellets along with their diet of fish.
Feathers get colours due to the presence of pigments, or by microscopic refractive structures, or by a combination of both. Most pigments are melanins (brown and beige pheomelanins, black and grey eumelanins) and carotenoids (red, yellow, orange); other pigments are found only in certain taxa – the yellow to red psittacofulvins (found in some parrots) and the red turacin and green turacoverdin (porphyrin pigments found only in turacos). Structural coloration is involved in the creation of blue colours, iridescence, and most ultraviolet reflectance and in the enrichment of pigmentary colours; structural iridescence has been reported in fossil feathers dating back 40 million years (4 crore). On the other hand white feathers do not have pigments and scatter light diffusely. There are certain birds that suffer from albinism, which is caused by flawed pigment production, but this does not affect structural coloration (as is evident in blue-and-white budgerigars). The blues and bright greens present in the plumage of many parrots are produced by constructive interference of light reflecting from diverse layers of the structures in feathers, in the case of green plumage in addition to the yellow pigments. The specific feather structure involved is sometimes called the Dyck texture. Melanin is often involved in the absorption of some of the light; in combination with yellow pigment it produces dull olive-greens.
Feather colours in some birds, may be created or changed by uropygial gland secretions, which may also have an inhibitory effect on feather bacteria. In many hornbill species yellow colour of the bill is produced by preen gland secretions.
Avian feathers suffer wear and tear and are replaced from time to time during the bird’s life through molting. New feathers, which are also known as blood, or pin feathers (depending on the stage of growth) when developing, are formed through the same follicle from which the old ones were fledged. The existence of melanin in feathers enhances their resistance to abrasion. The evolution of coloration is based on sexual selection and it has been suggested that carotenoid-based pigments may have evolved since they are likely to be more honest signals of fitness because they are derived from special diets, or because carotenoids are also required for immune function.
With such a variety and large number comes the problem of feather care that includes removing of dirt and parasites and re-oiling of feathers, which also provide efficient insulation besides giving body its shape. In birds the body temperature is much higher as compared to humans and it is also imperative for them to maintain it, which is done by trapping a layer of air within their plumage. To achieve this it is naturally very important that each feather should be kept healthy and well smoothened.
Birds keep themselves clean by preening, anting, bathing and moulting. Among all the above methods preening is very common. It provides much-needed Vitamin D to birds and also helps them in getting rid of the dirt and irritating parasites like fleas and lice. To preen, birds dip their bill-tip in the oil-like substance oozing from the preen gland, situated near the tail and then run their beak over the feathers. This not only oils the feathers making them waterproof, but also keeps them supple and smoothes the ruffled ones.
When this oil with each cleaning is exposed to sunlight, the vitamin is synthesized and can then be absorbed orally with the next bout of preening. The importance of the oil gland is extremely enhanced in the aquatic species that spend most of their time in water, so they possess the largest glands. Birds that have suffered damage to their oil glands are found to develop vitamin deficiency disease rickets.
For the purpose of keeping clean bathing in water, dust and sand is also very common, but there are some birds that even prefer sun, ants and the smoke. Ant-bathing is the most fascinating because it gives a kind of intoxicating kick to the birds. While anting birds first settle down on the swarming ant colony, ruffle their feathers and spread out wings encouraging the angry ants to crawl all over them. Some go even further picking up individual ants in their bills and wipe them along their feathers. It is believed that birds actually apply formic acid, produced by ants, to their plumage as a method of ridding themselves of irritating skin parasites.
As far as smoke bathing is concerned members of crow family have been observed sitting on the top of the chimneys with spread out wings and enjoying the smoke. Bathing (in water or beating their wings in dust) and molting (most birds at least once a year shed their old feathers and grow new ones) are also considered as a part of cleaning process.
Many water-birds, like ducks and geese etc., shed their flight feathers at once and become almost flightless for about three to six weeks. This is the time when they are most vulnerable, so they keep themselves concealed during the period and forage on water. All the birds belonging to this group, however, do not shed all their feathers at the same time; for instance, males (drakes) that do not take part in rearing of chicks moult sooner than females (ducks) that moult after the chicks have reached maturity.
Unlike the ducks, in swans both parents share the duty of looking after offspring, but it is the female that sheds her feathers first, and males take the responsibility of protecting the young. After her it is the turn of male to moult and his feathers attain full length at the same time as those of cygnets. This arrangement enables the whole swan family to migrate together.