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Essay on the Birds
Essay Contents:
- Essay on the Introduction to Birds
- Essay on the Origin of Flight in Birds
- Essay on Birds are Glorified Reptiles
- Essay on Birds as a Flying Machine
Essay # 1. Introduction to Birds:
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Birds constitute a very specialised group of vertebrates which have evolved from reptiles during Mesozoic era. The actual documentary records of the stages showing how such reptiles were transformed into birds are rare because of the non-availability of abundant avian fossils. Luckily a transitional stage between reptiles and aves is represented by Archaeopteryx and Archaeornithes, the two fossils from Germany.
These combine in themselves many of the reptilian features with undoubted avian features, so that the reptilian origin of birds is obvious. The gap between this stage and the actual reptilian ancestor is yet to be filled. Two fossil specimens of Archaeopteryx discovered from the Upper Jurassic beds added the strongest evidence to this contention.
The exact nature of hypothetical ancestor ‘Proaves’ which formed the link between Archaeopteryx and the reptile has been a matter of speculation. Some of the theories are mentioned here as they are of interest.
According to Osborne, the ancestral bird was an arboreal four-legged parachuting animal which glided from tree to tree like a flying squirrel. The parachute became an active wing with the development of feathers and strengthening of wing muscles. In other flying animals like bats and pterodactyls with patagium, the hindlimbs are incapacitated for walking, whereas in birds, they are prominent.
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Nopsca put forward the theory that the ‘Proaves’ was a cursorial long-tailed, bipedal creature which ran about and leaped along the ground with their strong hindlimbs, flapping their forelimbs in the air as an aid to increased speed. The forelimbs which were the wings became broadened by the development of scales in the hinder part of the arm and the scales became sprayed out to form quill feathers.
Steiner suggests that the ‘Proaves’ should have been an arboreal form with four legs, the hindlegs being modified for springing, and forelegs for climbing and steadying the body after a leap. The quill feathers developed along the hinder aspect of the forelimbs and the tail which helped the animal to buoy up the body in the air. The transformation of the forelimbs into wings was accompanied by the development of bipedalism.
Beebe suggests that the Proaves should have been an arboreal tetrapod with backwardly directed feathers in all the legs, both sets serving as parachutes in gliding. This view is supported by indications of tufts of feathers in the hinder aspect of the limbs of Archaeopteryx.
Gregory is of the view that birds have a dual origin, some have evolved from cursorial and others from arboreal ancestors, which will be seen to be a compromise of several theories.
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Presently the origin of birds is considered mainly on two lines:
a. Diphyletic origin and
b. Monophyletic origin.
a. Diphyletic Origin of Birds:
The earliest known fossil birds include both flying (Archaeopteryx, Ichthyornis) as well as flightless (Hesperornis, Diatryma) types. The recently extinct Moas and Elephant birds were also flightless. The most primitive living birds or Ratitae (Ostrich, Rhea, Cassowary, Emu and Kiwi) and Penguins are also flightless.
This led some authors, notably P. R. Lowe, to believe in the diphyletic (two-lines-of-descent) origin of birds. They maintain that the flightless and flying birds of today have descended from flightless ancestors. According to Lowe, the present-day flightless birds were never capable of flight, and their wings are not degenerate now, but better developed than any time in their past history.
b. Monophyletic Origin of Birds:
In Ratitae, the legs are well-developed and powerful, the wings vestigial, and the feathers are fluffy. But a recently discovered fossil of Eleuherornis, a probable ancestor of the present- day ostrich from the Eocene of Switzerland, shows closer affinities to flying forms than does the present-day ostrich and poses a serious blow to the concept of diphyletic origin of birds.
Presently most palaeontologists believe that the Carinatae are more primitive. Presumably the Ratitae evolved from flying ancestors but readapted to a terrestrial mode of life in areas with abundant food and few competitors or enemies.
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The more usually accepted view today maintains that birds have a monophyletic (one-line-of-descent) origin, i.e., all birds have evolved from a single ancestor, perhaps close to Archaeopteryx. Accordingly, the flightless birds have evolved by loss of flight from flying ancestors. The weight of the known evidence also favours this view.
Essay # 2. Origin of Flight in Birds:
The evolution of the birds is associated with the changes from terrestrial to aerial life. The reptilian origin of birds is a self-evident fact, but detailed evidence of the steps how cold-blooded terrestrial reptiles were converted into warm-blooded flying birds are insufficiently known except the discovery of a fossil bird, Archaeopteryx, in the Jurassic bed.
We know nothing about the evolution of feathers from reptilian scales, although intermediate structures between scales and feathers are present on the legs of ostrich and fowl. We, of course, do not know just how flight evolved in the ancestors of Archaeopteryx.
It is presumed that the ancestors were becoming more active and possibly warm-blooded and feathers developed from scales primarily to conserve their body heat. Later, the feathers enlarged on the limbs and that the tail probably to provide stability in fast running on ground or in rudimentary gliding from branches.
Different theories have been proposed to explain the origin of flight in birds, starting from either a terrestrial, bipedal or cursorily ancestor (Nopcsa) or an arboreal ancestor (Osborn, Steiner, Beebe, etc.).
1. Theory of Cursorial Origin of Flight:
Nopcsa has advocated the cursorial origin of flight. According to him, the proaves was a running reptile. While running fast on the ground, this reptile lifted the forelimbs off from the ground. The scales over the margin of the limbs became expanded as feathers.
Nopcsa believed that birds arose from bipedal dinosaur-like reptiles which used to run on the ground by hindlimbs. According to his theory, the ancestors of birds were long-tailed, cursorial, bipedal animals.
They were fast runners who leaped on their strong hindlimbs and flapped their forelimbs in air to help them along, as do many modern birds that run fast. Gradually the forelimbs enlarged due to fraying out or elongation of scales forming quill-feathers through the process of mutation and selection. In the end, the forelimbs became organs of flight or wings rather than accessories to rapid running.
2. Theory of Arboreal Origin of Flight:
This theory of arboreal origin of flight postulates that the ancestral birds were arboreal creatures. This concept is supported by many ornithologists. Proves was arboreal form which used to live on trees. They climbed trees from which they glided to the ground or to other trees, like the modern flying squirrels. Osborn has put forward the pair-wing theory and advocated that only the forelimbs were utilised in climbing the trees and also during jumping from one tree to another.
This has led to the origin of patagia between the limbs and the body. Beebe and Heilmann established the four-wing theory which signifies that both the forelimbs and hindlimbs were need during the process. Both the pairs of the limbs were utilised for climbing the trees and also during parachuting from tree to the ground and from tree to tree.
As time went on, the forelimbs gradually enlarged and eventually converted into wings which were able to support the animal in flight. The feathers of the hindlimbs vanished and the tail shortened as seen in the modern birds. Thus, flight was an outgrowth of climbing and gliding.
Steiner maintains that the hindlimbs of the ancestor were modified for springing, while the forelimbs were used for climbing and balancing the body after a leap in the air.
3. Compromise Theory of Origin of Flight:
Gregory advanced a compromise idea and believed in the dual origin of birds. Believers in the dual origin of birds maintain that some birds evolved from arboreal and others from cursorial ancestors.
4. Diving Theory of Origin of Flight:
Newmann regarded the proaves as aquatic reptile. Flight might have started in connection with soaring over the water during diving for fishes.
Essay # 3. Birds are Glorified Reptiles:
Nearly a century ago, T.H. Huxley called birds ‘glorified reptiles’, thereby meaning that birds have evolved from some reptilian ancestor and that they are better evolved in their organisation. The evidence from comparative anatomy, embryology and paleontology with fossil Archaeopteiyx as the transitional form suggests that bird is a highly specialised descendant of some reptilian ancestor.
The steps and the impulse under which evolution of birds took place from a reptilian stock are not properly understood at present. Whatever may be mode and direction of origin of birds from ancestral reptiles, the modern birds show a marked superiority over their reptilian ancestors.
Some of the main changes leading to the glorification or superiority of birds over reptiles are as follows:
1. The reptiles are generally sluggish, cold-blooded (poikilothermous) and earth-bound, the birds are active, warn-blooded (homoiothermous) and alert, having more life in them than any other living creature. They have a higher rate of metabolism and correlated carefully regulated high body temperature. They are warm-blooded and due to this they are equally active throughout the year.
2. Birds show a more rapid locomotion due to power of flight, perhaps also the most efficient in the animal kingdom.
3. The development of avian feathers from reptilian scales is definitely an advance. It has made it possible for birds to fly. The insulating feather covering preserves body heat and makes the birds warm-blooded. The elongation of feathers on forelimbs has modified them into wings for flight. Thus, the variety and colouration of various types of feathers show an advance over the simple uniform type of scales in reptiles.
4. Birds, like mammals, have a completely four-chambered heart with double circulation, in which there is no mixing of pure and impure bloods. Only a single systemic arch that of the right side, persists in birds, whereas both the systemic arches are present in reptiles.
5. The renal portal system is well developed in reptiles, while vestigial in birds. It is absent in mammals.
6. In birds respiratory system is highly evolved. The lungs, with the development of air sacs, bring about complete aeration and are many times more effective than those of reptiles.
7. Birds have developed the voice with all the varied features of calls and songs, in contrast to the silent reptiles.
8. Kidneys are metanephric, but show two specialisations not found in reptiles. First, the uriniferous tubules are relatively longer, and second, a U-shaped piece, called the loop of Henle, is usually inserted in their middle. This condition is similar to that of mammals.
9. The endoskeletal, muscular, alimentary and reproductive organs also show an advance over those of reptiles.
10. The brain is proportionately bigger with better development of cerebrum and cerebellum. The sense of sight and hearing are better developed. Eyes are more efficient with better power of accommodation. Ears possess cochlea with an organ of Corti and, therefore, are better developed.
11. Bird’s eggs resemble those of reptiles but differ in that many of them are coloured.
12. Birds possess higher grades of intelligence and behaviour practically unknown in reptiles, such as :
(i) Parental care and extreme emotion in preparation of more or less elaborate nest,
(ii) While most reptiles leave their eggs to hatch in ordinary environmental conditions, all birds incubate their eggs, thus ensuring hatching and better survival,
(iii) Periodic migrations or wandering to and fro their winter and summer abodes,
(iv) Monogamous life with selected mating, courtship behaviour and affection for the mate and
(v) Singing and mating calls during breeding season.
Thus, Huxley was fully justified in commenting that ‘birds are glorified reptiles’.
Essay # 4. Birds as a Flying Machine:
The flight is a spontaneous action. At take-off the bird has to acquire sufficient forward momentum to provide lift. In many birds the jump is provided by the legs, which is adequate for the takeoff. Larger or heavier birds such as peacock run or swim rapidly to gather enough forward momentum for a take-off many large birds nest on a cliff or tree which gives them an up-current for the take-off. Small-sized birds such as pigeons usually take a quick jump by means of their legs followed by the beating of their wings.
The pigeons and other birds on the principle of aeroplane or heavier than air machine. Its body is well adapted for aerial mode of life.
Such flight adaptations include the following ways:
1. Centralisation:
The internal organs are well centralised and well adapted for their mode of life. For example, presence of light horny beak without teeth and muscular gizzard having stones for grinding the seeds. Tail is short and provided with tail feathers as rudder during flight. The head is small and light, and the neck is long which can be retracted during flight. The body axis is short and the body is streamlined and perfectly balanced with centre of gravity well below the wings.
2. Lightness and Rigidity:
The bones are light, strong and pneumatic built on a hollow girder principle.
3. Wings for Support:
The large, broad wings and tail provide support during flight. Birds are able to adjust their wings and tail during flight in the fast flowing air.
4. Sustained Power:
Birds have a great sustained power of flight. This is attained by the well-developed flight muscles, large heart allows a greater stroke output and higher pressure than in mammals, large-sized red blood corpuscles which carry a large amount of haemoglobin that gives up its oxygen suddenly at a relatively high oxygen tension and high basal metabolic rate and high body temperature (42°C- 45°C). High body temperature and great activity of birds necessitate a high food intake. This is made possible by rapid passage of food through the gut.
5. Steering and Balancing:
This is done by the help of wings and tail. Wings are the active means of flight. The wings support the body weight in air. Depression of the wing elevates and propels the bird through air. Rotational movements of the wing also help the bird during flight.
Thus, wings act as balancing and flying organs. The tail with tail feathers acts as a rudder for steering during flight, it suddenly checks flight and as a counterbalance in perching. It also directs the course of flight by spreading or folding and elevating or depressing the tail.
Mechanism of Flight:
The pigeon and other birds fly on the principle of indirect movement. They move the air, which, by its displacement, moves the birds. Air displaced by the beating of wings, sets up currents that keep the animal aloft and move forward, resulting in flight. A plane surface moved through the air in a direction inclined at an angle to its plane is called aerofoil.
The forces generated can be resolved into a lift force acting upwards and a drag force to stop the motion. On this fact depends the power of supporting weight in the air that is possessed by birds. Both lift and drag forces are proportional to the square of the speed and the requirement for sustained flight in still air is that the object shall have sufficient speed to generate a lift force equal to its weight.
The flow of air over the upper surface of the wing reduces the pressure there and provides the main portion of the lift.
By tilting the wing the pressure on the underside can be increased, but the air flow becomes turbulent especially at the hind edge destroying the lift. When an aerofoil fails below the critical speed, it stalls (drops suddenly) being no longer supported. The smooth flow of air over the wing disturbs the wings’ hinder (trailing) edge and by eddies round the end (tip vortex).
A small wing area is necessary for fast flight, at least for high speeds. Wing provides the forward momentum as well as the lift. For flapping flight the wings must be moved relatively fast and for this small size is an advantage. While a large wing area allows slow flight. Although a small wing area reduces drag, many fast flying birds have a large wing-span. It allows a slow rate of descent when gliding, reducing the expenditure of energy necessary to sustain flight.
A pointed wing tends to stall first at its tip and is only suitable for fast fliers. Whereas birds with shorter broader wing possess slower flight.
The condition of the air around the wing is of first importance for maintenance of lift. If there is not a smooth stream over the upper and under surfaces, the air becomes turbulent, and the aerofoil stalls. This tends to happen either if the speed falls too low or if the angle of the wing relative to the line of motion increases above 20°. Turbulence is mitigated by the openings (slots) which let through part of the air and provide the necessary smooth stream.
The spaces that occur between the feathers, especially towards the wing tip junction as slots. It provides a very efficient high life device. Such slots are conspicuous in slow fliers (rooks) and especially in those that soar on thermal up-currents (vultures). Slots are also found in the wings of large fast-flier birds (pheasants). In most birds there is a thick loading edge and a thinner trailing edge.
Nearly all wings are convexly curved in cross section, especially in the region of forearm. This arrangement directs the air stream over the upper surface of the wing in such a way as to provide an extra lift by creating a suction zone of reduced pressure. Highly convexly curved wings and less wing area reduces the speed of the bird.
In some birds feathers are allowed to twist only when the wing is being raised and the barbs of the feathers open like the vanes of a blind when under pressure from above, but close when the pressure is from below. In fast flying birds with a slow wing beat (gulls and swans) the wing is probably rigid on the up as well as on down strokes, and is twisted so as to produce forward and upward components on the upstroke.
The whole upward movement is usually faster than the downward one. Before the wing tip has reached its highest point, the upper arm is already beginning to descend and in this way the line of flight is maintained almost straight and does not follow a wavy path as it would do if the parts of the wing vibrated together.
In small birds the wing works more nearly as a whole and the flight differs in several respects from that of larger birds. In general the wing is a very labile system and regulates itself automatically with changes in the aerodynamical forces. This regulation is produced partly by feather plasticity and joint mobility, with participation of reflex muscular adjustments that are little understood.