THE BIOLOGY OF BIRDS

The three classes of living animals which have made a sucoess of flying are birds, bats and insects, and of these the birds are the most highly specialized, for almost every part of their bodies is adapted in some way or other for life in the air. A fourth type of flying animal, known as the pterodactyl, existed millions of years ago in tertiary times, but it became extinct owing to change of climate. The pterodactyl was a reptile which flew by means of a web of skin supported by the very long fourth finger of the fore limb, and which was joined to the hind leg. But although birds are believed to have descended from reptiles and still show many reptilian features, they are not related to the pterodactyls. It is thought that they may have evolved from a group of extinct reptiles known as dinosaurs, some of which ran on two legs. Possibly these reptiles took flying leaps which were the beginning of a simple gliding type of flight.

The Shape of the Body

Most flying birds have comparatively small lightly built bodies, for it is obvious that in a flying animal any weight reduction which docs not mean a loss in power and strength will be an advantage. Some flightless birds, such as the ostrich and emu, have large heavy bodies, but this is only possible because these birds have lost the power of flight and taken to running on the ground. The flying bird has a streamlined body, pointed at either end and broadest in the middle, so that it can move through the air with the least effort.

A bird ’s feathers overlap and point backwards so that they are not ruffled by the wind, and during flight the feet are held up so that they offer the least resistance to passage through the air. The pigeon and many other birds, for example the blackbird, thrush and the swallow, tuck the feet up close to the body, while the duck and heron thrust them out behind parallel with the tail.

Feathers

The possession of feathers is a characteristic feature which distinguishes birds from all other vertebrates. Feathers really represent the scales which cover a reptile, but which have become much larger and have frayed out at the edges. They form a light but efficient covering which keeps the body warm and also prevents it from becoming wet, since raindrops run off the feathers. Birds keep their feathers in order by preening them with their beaks, which occasionally may be seen reaching towards an oil gland situated on the upper side of the hind end of the body, and then smoothing the feathers down with the secretion obtained. It has been suggested that this secretion waterproofs the feathers, although according to one authority the removal of the preen gland makes no difference to the bird. The feathers are not uniformly distributed over the whole body, but occur in certain tracts, and the bare patches in between these are covered by the overlapping of the feathers.

A typical feather , such as the wing feather of a fowl or pigeon, consists of a central rod which bears a flat expanded part called the vane. The lower part of the central rod is hollow and forms the quill ; this has a hole at the base into which a small plug or papilla of tissue fits. The base of the quill is sunk into a small pit in the skin called the follicle. The quill is continuous with the solid upper part of the central rod, which is called the shaft or rachis and which bears the vane. If the vane is ruffled with the fingers it is seen to be made up of a great many fine processes, called barbs, arranged on either side of the rachis like the teeth of a comb, but set obliquely. Each barb bears on each side a row of small branches called barbules, which are also arranged obliquely, so that the barbules of adjacent barbs overlap. It is by means of the bar bules that the barbs are held together to form the firm structure of the vane. Microscopic examination of a single barb shows that the barbules on the side towards the tip of the feather are provided with very small hooks, while those on the opposite side bear ridges. The barbule hooks of one barb interlock with the barbule ridges of the next barb, so as to bind the whole of the vane together and provide a surface capable of beating the air.

The feathers which cover the body have a similar structure to the quill feathers and are known as covert feathers, but besides this type of feather there are two other kinds. In many birds down feathers occur in between the covert feathers, and are characterized by a short shaft with barbs which do not interlock. The young birds of many species have a temporary covering of down feathers. The third type of feather is known as a filoplume, and is found all over the body in between the covert feathers. The filoplumes are the small feathers which have to be burnt off when a fowl has been plucked, and each consists of a slender thread-like shaft at the end of which is a tuft of barbs.

All birds moult once a year, and in this way replace feathers which have become worn or broken, for feathers consist of dead cells and cannot effect their own repairs. The moulting is generally spread over a considerable period.

The Wing

The fore limb of a bird is modified so that it performs the function of both the wings and the propeller of an aeroplane. The three parts of the limb corresponding to the upper arm, forearm and hand of a mammal can be recognized, and when the bird is not flying these parts are neatly folded up in the form of a letter Z ; in this position they fit closely to the body so as to form no hindrance when the bird is walking or hopping. When the wing is expanded the upper arm, forearm and hand are prevented from lying in one straight line by two folds of skin, one of which extends from the upper arm to the wrist and the other across the arm-pit. The greater part of the wing surface is formed by the feathers, which provide a large striking surface and yet do not greatly increase the weight of the wing. The muscle and bone of the wing form a light framework to the hinder edge of which the quill feathers are attached. The weight of the wing is further decreased by the reduction in the number of digits, of which only three are present. The thumb only remains as a separate digit, and the second and third digits are immovably enclosed in a web of skin. The thumb bears a small tuft of feathers, while the large quill feathers are attached to the hinder border of the hand and forearm. Those fixed to the hand are known as primaries and those fixed to the forearm as secondaries. The air is prevented from passing through the bases of the quill feathers by several rows of small feathers, known as wing coverts, which occur on the upper and lower sides of the wings.

Flight

When extended a bird ’s wings are not perfectly flat but are curved in the same way as the wing of an aeroplane, with a convex upper and a concave lower surface. During flight the wings are raised above the back and then pulled downwards, and the effect of the compression of the air by the cambered surface of the wing is to carry the bird forwards and upwards. Actually flight is not such a simple matter as this, for besides the up and down movement of the wing there are twisting movements, and the tip of the wing moves in a figure of eight. During the upstroke the air resistance is minimized by the convex upper surface of the wing which allows the air to glide off easily, and by a movement of the quill feathers which allows the air to pass between them. The tail feathers help the bird to balance and are used to some extent in steering, but this is generally effected by the distal part of the wings. If, for instance, a bird wishes to turn to the right it beats more forcefully with the hand part of the left wing. On landing the tail is spread out upwards and acts as a brake. Many birds, such as seagulls, are PeWls expert gliders and are able to make use of air currents, so that they can float about in the air for a long time, with only an occasional flap of the Anhle ½ °int-wings to stop them from sinking.

The Legs

Since the fore legs are modified to form wings, a bird has to balance its body on two legs, and many of the skeletal features are related to this posture. A bird. walks on its toes with its ankles lifted off the ground, and the leg is not held straight but bent so that the knee points forward and the ankle backwards. This arrangement makes it easier for the bird to keep its balance, and enables it to spring easily from the ground when taking to flight. It also lessens the shock on landing. The thigh bone lies hidden in the body and the shin is covered with feathers, while the feet are covered with scales as far as the ankle. In most birds three toes are directed forward and one backward, so that they are able to grip a support while perching,

Owing to the arrangement of the tendons, the toes are automatically curved when the bird perches. The tendons pass to the toes along the back of the shin and behind the heel. When a bird perches it settles down so that the leg becomes more bent, and consequently the tendons are tightened and the toes grip the support. The lower the bird sinks down on its perch, the tighter the toes grip. This arrangement prevents the bird falling off its perch when asleep. Before a bird can relax its hold on the perch, it must straighten its legs so as to lessen the tension on the tendons causing the toes to grip. In parrots and woodpeckers two toes are turned backwards, while owls are able to turn the fourth toe in either direction.

The Feeding of Birds

The jaws of a bird are pointed and covered with a horny sheath so as to form a beak, which shows many adaptations in connection with the kind of food. The rook and blackbird are omnivorous feeders and have strong pointed beaks which are not specialized for any particular kind of diet. Flesh-eating birds, such as the eagle, hawk and owl, have sharp hooked beaks suitable for killing their prey and for tearing strips of flesh from the victim. The parrot also has a hooked beak, but in this case the beak is used for cracking nuts and extracting the inside. Others, seed eaters, such as the chaffinch and sparrow, have short, strong, conical beaks which are suitable for picking up small seeds. Insect eaters, such as the woodpecker, have stout beaks which can pick insects from cracks in the bark of trees and other crevices. Swallows and swifts also feed on insects, but in these birds the beak is short, and when open presents a wide gape, so that they can catch their food while on the wing. The heron has a long, strongly made beak with which it spears fish, while the curlew and snipe have a long narrow beak used for probing earth and mud for the worms and grubs therein. The inside of a duck ’s beak is grooved and ridged so that a slippery animal, such as a frog, is securely held. When dabbling in water the duck will take a bcakful of muddy water, and then squeeze it out along the sides of the beak through the sieve formed by the ridges, retaining anything edible.

In many birds the feet show adaptation in connection with the food supply. Birds of prey have long sharp claws with which the prey is seized, and have a very strong grip which prevents the escape of the victim. Poultry and game birds have strong feet and blunt nails for scratching in earth to find seeds and worms. In many water birds the feet are so modified as to act as paddles. In ducks the feet are placed nearer the hind end of the body than in non-swimming birds, and the legs are widely separated from each other, so that these birds waddle and roll when they walk. The toes are united by webs of skin, and are stretched apart during the backward swimming stroke, but are closed together during the forward stroke so as to offer less resistance to the water. Resistance to the water is also lessened during the forward stroke owing to the fact that the bone running from the toes to the ankle is flattened and moved edge fonvards. During the backward stroke this bone is partly rotated so that the flattened side adds power to the stroke. Penguins swim by means of their flipper-like wings, which are not used for flying. Their wings are covered with small, flattened and very oily feathers.

The Sense Organs of Birds

The hands are so modified for flight that they are useless for other purposes, and the function of a hand is taken on by the beak. The neck is very flexible and can be bent in any direction so that the bird can preen the feathers on all parts of its body. The nostrils are two small slits on the upper part of the beak, but the power of smell is poorly developed in birds, which have, however, a very keen sense of sight and hearing. The eyes are large and are protected by two eyelids and also by a thin transparent membrane called the nictitating membrane. This passes across the eye from the front corner from time to time, and helps to keep the eye clean. The ear opening occurs behind the eye and there is no external ear-lobe, but a circular patch of small feathers protects the opening. The opening leads into a short tube, at the bottom of which is the ear-drum.

Reproduction of Birds

As is well known, birds reproduce by laying eggs, as described in the previous section on reptiles, with which they have a number of features in common. Usually the eggs are laid in nests and incubated by the hen bird. After hatching, the young are fed and tended by the parents until able to fend for themselves.

Similarities between Reptiles and Birds

Both lay eggs, usually with calcareous shells.

Both have epidermal scales.

BIOLOGY OF MAMMALS MAMMALS

THE class of mammals which are the highest of the vertebrates includes the four-footed beasts, such as the cat, dog, cow, horse and sheep, which the average person terms animals. Such common wild animals as the lion, tiger, leopard, and bear are also mammals. All these move on four legs by walking or running, whilst the kangaroo hops on two legs and the whale uses its fore limbs for swimming. There are also flying mammals, for example the bat, and climbing mammals, such as the monkey. Finally man himself is placed in this class of vertebrates. Although mammals have not reached the high level of specialization in general structure shown by birds, yet they are outstanding among vertebrates as animals which have the most highly developed brain, and which show the most skilled use of it. Most animals attack a new problem or situation in their lives by making many haphazard attempts to solve it, and eventually hitting upon the correct solution by a chance effort which is sucoessful. Further, most animals learn by this trial and error method ; they remember which efforts are followed by satisfactory results and which by harmful results. Mammals, too, learn by trial and error, but they learn more quickly and remember more sucoessfully than other animals. A man faced with a new problem tries to understand it, and compares it with any other similar situation which he has experienced before he attempts to solve it. A few other mammals, in particular the chimpanzee, appear to do this.

Like birds, mammals are warm blooded, and there is evidence to show that they also have descended from reptiles, but along a very different line from birds. They are distinguished from birds and other vertebrates by many well-marked features, of which the following are the more important:—

Hair. /

Sweat glands.

Teeth which differ in shape and function.

A diaphragm.

A large and complicated brain in which two parts, the cerebrum and cerebellum, are very large.

Mammary glands.

Viviparous reproduction and the Spiny Ant-eater.

Hair

Birds have a covering of feathers, but mammals are characterized by the possession of hair. The hairs may form a close covering for the body, as in the sheep, or may be represented by a few bristles, as in some kinds of whale. A hair is a solid rod of cells which grows out of a tube in the skin called the hair follicle. There is a slight hollow at the base of the hair, and into this fits a small plug of tissue termed the hair papilla. The papilla contains blood capillaries which bring materials for the construction of the growing hair. Each hair is kept oiled by a secretion from a small gland, the sebaceous gland, which opens into the follicle near the top. The hair generally lies at an angle to the skin, but when the animal is cold or frightened it stands on end. This movement is caused by the contraction of a small muscle attached to the base of the follicle. The value of a covering of hair for keeping in the heat lies in the fact that next to the skin there is always a layer of air, which is prevented from being blown away and mixing with the outer atmosphere by the hairs. Since air is a bad conductor of heat, the stationary layer of air next to the skin helps to prevent the loss of heat from the body, I.e. is in effect an air blanket. When it is very cold the hair of a mammal stands on end, so that its coat becomes fluffed out; in this way the thickness of the layer of stationary air is increased, and the chance of heat loss is further cut down. The goose-flesh that we get when we are cold is an unsucoessful attempt by the skin to prevent heat loss in the same way, which gives evidence of a more hairy ancestry. Just as the feathers of a bird are lost when it is moulting, the hairs of a mammal drop out and are replaced by new ones. Often this takes place so gradually that no difference is noticed. But in many mammals the winter coat is much thicker and longer than the summer covering, and is sometimes a different colour. Hair colour is due to pigment in the cells of the hair, but white hair is caused by the presence of minute air bubbles in between the hair cells.

Sweat Glands

These are long coiled tubes with walls one cell thick lying beneath the epidermis, and which open by small pores on the surface of the skin. Sweat is produced by the cells of the tube, and consists mainly of water containing urea and common salt. By its evaporation the sweat helps to cool the skin, and at the same time it helps to rid the body of small quantities of excretory material.

The Teeth of Mammals

The shape and number of the teeth in mammals show great diversity, and are related to the kind of food that is eaten. Each tooth is sunk in a socket in the jawbone, and consists of three regions, namely :—

The crown or upper part of the tooth above the gum.

The neck, a slightly narrower part of the tooth between the crown and the jawbone.

The root, which is the part sunk in the socket of the jaw.

The greater part of the tooth consists of a hard substance, dentine. This surrounds a central pulp cavity in which is living tissue containing nerves and blood vessels. The dentine of the crown of the tooth is coated with a layer of extremely hard substance termed enamel. The root of the tooth is coated with cement, a substance like bone, and a membrane which fixes the tooth in its socket. In most cases, at a certain size the growth of a tooth ceases and the hole at the base of the root leading into the pulp cavity becomes very small. In such a tooth the root is said to be closed. In some animals, however, the tooth continues to grow, and the root has a wide opening into the pulp cavity and is said to be open. Such teeth, which grow as fast as they are worn away, are found in herbivorous animals such as the rabbit, sheep and horse.

Mammals have two sets or dentitions of teeth during their lives : the first set is the milk dentition which lasts for a short time while the animal is young, and is followed by the permanent dentition which lasts through life. The milk teeth are smaller in size and fewer in number than the permanent teeth. The majority of mammals have teeth of four different kinds which in order from the front of the mouth to the back are :—

Incisors. These have sharp chisel-like edges for cutting off lumps of food.

Canines. These are pointed and are used for tearing. They are sometimes called ’eye teeth ’or ’dog teeth. ’

These are usually similar in shape, and have either one or more sharp points for cutting, or have flat tops for grinding. The difference between the molars and premolars lies in the fact that the molars are not present in the milk dentition.

The number and kinds of teeth in the mouth of an animal are conveniently shown by use of the dental formula, which represents the teeth in one half of the upper and lower jaws. The letters refer to the kind of tooth, and the figures to the number in upper and lower half-jaws.

The Teeth of a Dog

The dog may be taken as an example of a mammal with a carnivorous dentition.

The incisors are small and pointed, and behind them are the long, curved and pointed canines, which are used for tearing flesh and as weapons of offence and defence. The premolars have a compressed shape and have one or more sharp points. The last upper premolar and the first lower molar are particularly large, and are termed the carnassial teeth. These teeth are used for cutting flesh from bones, and when the dog bites they slide past each other with a scissor-like action. The lower carnassial bites within the upper one, and the lower jaw is so jointed to the skull that only up and down movement is possible and there is no play from side to side. The first upper molar is a broad tooth with an outer cutting edge of two cusps and an inner part with two or three blunt points. This inner part of the tooth has a crushing function and bites against the hinder part of the lower carnassial. The second upper and lower molars are smaller teeth, but also have blunt points used for crushing. The last lower molar is small and peg-like, and has no tooth to bite against in the upper jaw.

The Teeth of a Rabbit

The rabbit has two pairs of incisors in the upper jaw and a single pair in the lower jaw. The front pair of upper incisors are long and curved, and bite against the lower incisors, which are similar in appearance but are not so strongly curved. The layer of enamel is thicker at the front of the incisors than at the sides or back, so that the teeth wear away quicker at the back than at the front, and a chisel-like cutting edge is produced. The teeth grow continuously through life, and if one tooth is lost the opposing tooth grows until it is a long curved tusk, which may pierce the other jaw and cause such difficulty in feeding that death by starvation results. Sometimes when a lower incisor has been lost the corresponding upper incisor grows right round until it pierces the hard palate. The second upper incisors are small pegs which lie behind the first pair. There are no canines, and there is a wide space, the diastema, between the incisors and the premolars. The premolars and the molars have flat tops with ridged surfaces and are used for grinding. The last upper and lower molars are smaller and more simple than the others.

The rabbit feeds by nibbling vegetation with its incisors and then grinding up the food with its back teeth. The hairy sides of the cheeks project into the mouth between the incisors and the premolars, and together with the tongue help to pass the food back from the incisors to the grinding teeth. The articulating surfaces of the skull and lower jaw do not form a transverse joint as in the dog ’s skull, but are elongated in a direction parallel to the middle line of the skull. As in the case of the dog, this arrangement prevents lateral play, but in addition it allows a backward and forward movement of the lower jaw which greatly helps the gnawing action of the incisors. Rabbits and other animals with a similar dentition are placed in the order of mammals termed the Rodentia or gnawers.

The Teeth of Herbivorous Mammals

Such herbivorous animals as the horse, sheep and cow have the back teeth adapted for grinding up vegetation. These teeth have broad and flattened crowns on which ridges run transversely, and when chewing the animal moves its jaws from side to side.

The incisors of the horse become flat-topped with wear, and are used for gripping and tearing off the grass. The canines are short, bluntly pointed teeth, occurring a short distance behind the incisors, and are separated by a wide space from the premolars. The teeth of the sheep and cow are similar to those of a horse, but there are no incisors or canines in the upper jaw, but instead there is a hard pad against which the front teeth of the lower jaw bite.

The Teeth of Man

In the upper jaw there are two pairs of incisors, which are broad teeth with sharp cutting edges. The canines are bluntly pointed, and behind them lie the premolars, which have two blunt cusps and usually a single root. Each molar in the upper jaw has three or four cusps and usually three roots. The teeth of the lower jaw are similar to those of the upper jaw, except that the molars have two roots instead of three. The last molar teeth of each jaw are the wisdom teeth, so called because they do not appear until the individual is 17 to 25 years old. Sometimes the wisdom teeth do not appear at all.

The Diaphragm

The mammals are the only vertebrates which possess a diaphragm. This is the muscular partition which divides the coelom into the chest cavity and the abdominal cavity.

The Brain

The brain of mammals is more highly organized than that of any other animals, and two parts, the cerebral hemispheres and the cerebellum, have become very large.

Reproduction

Two primitive mammals lay eggs, but other mammals give birth to their young, which develop in a cavity inside the mother ’s body called the uterus. The period of development within the uterus is called the gestation, and it varies greatly in length in different mammals. In the case of pouched mammals such as the kangaroo and opossum, the young are born in an imperfect and exceedingly helpless condition, and undergo a long period of childhood. Except in the pouched mammals the embryo is joined to the tissue of the uterus by a structure called the placenta, through which oxygen and nutritive substances pass from the blood of the mother to the embryo. From the embryo carbon dioxide and other waste products are passed back to the mother. After birth the offspring of mammals are fed for a time on milk, which is a secretion produced by the mammary glands. Thus not only is the young mammal protected within the body of the mother before birth, but after birth it is fed and protected by the mother, and in many cases educated as well.

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