Birds, an Introduction
For illustrations to accompany this article see Birds, Structure & Function
Birds are ‘warm-blooded’ vertebrates, with fore-limbs modified to wings, and skins covered with feathers. Vertebrates are characterised by having a spinal column and a skull. ‘Warm blooded’ or homoiothermic (constant temperature) means that their body temperature is kept more or less constant and above that of their surroundings. Typically, the forelimbs as wings give birds the power of flight although there are some flightless birds. In some cases (e.g. penguins and puffins) the wings are used for swimming under water.
All birds reproduce by laying eggs which are fertilised internally before laying.
The skull and lower jaw are extended forward into mandibles which make a beak.
The bird's legs and toes are covered with overlapping scales.
Birds possess a third, transparent eyelid, the nictitating membrane, which can move across the eye.
The feathers are the single external feature that distinguish birds from other vertebrates. The feathers are produced from the skin which is loose and dry, without sweat glands, and they form an insulating layer round the bird's body, helping to keep its temperature constant, and repelling water. The wings are specially developed for flight, having a large surface area and very little weight.
The barbules of the feathers interlock in such a way that should a feather be damaged in flight, preening with the beak will re-form it perfectly.
The feather quills have attached to them muscles which can alter the angles of the feathers; for example, when a bird fluffs its feathers out in cold weather. They also have a nerve supply which, when the feathers are touched, is stimulated in a similar way to a cat's whiskers.
The down feathers are fluffy, trapping a layer of air close to the body. The flight feathers and coverts are broad and flat and offer resistance to the passage of air.
The shape of the bird and the lay of its feathers make it streamlined in flight.
Features which adapt the bird for flying
1. The fore-limbs are wings with a large surface area provided by feathers. However, rather than being an ‘adaptation to flight’ they are essential for flight to take place.
2. Large pectoral muscles for depressing the wings. They may account for as much as one-fifth of the body weight in some birds.
3. A deep, keel-like extension from the sternum (breast bone) provides for the attachment of the pectoral muscles. Well-developed coracoid bones transmit the lift of the wings to the body.
4. A rigid skeleton giving a firm framework for attachment of muscles concerned with flying movements. Many of the bones which can move in mammals are fused together in birds; for example, the vertebrae of the spinal column in the body region.
5. Hollow bones, which reduce the bird's weight.
The flight of a bird can be divided into flapping, and gliding or soaring, different species of birds using the two types to varying extents. In flapping flight the pectoralis major muscle contracts, pulling the fore-limb down. The resistance of the air to the wing produces an upward reaction on the wing. This force is transmitted through the coracoid bones to the sternum and so acts through the bird's centre of gravity, lifting it as a whole.
In addition to the lift, forward momentum is provided by the slicing action of the wing, particularly near the tip. In the down-stroke the leading edge is below the trailing edge so that the air is thrust backwards and the bird moves forward. The secondary feathers provide much of the lifting force and the primaries most of the forward component.
The bastard wing (a group of feathers attached to the first digit) may be important during take-off for giving a forward thrust. During flight it may function as a slot maintaining a smooth flow of air over the wing surface.
The up-stroke of the wing is much more rapid than the down-stroke. The pectoralis minor muscle contracts and raises the wing, since its tendon passes over a groove in the coracoid to the upper side of the humerus. Often the arm is simply rotated slightly so that the leading edge is higher than the trailing edge and the rush of air lifts the wing. The wing is bent at the wrist during the up-stroke thus reducing the resistance. In addition, the way in which the primary and secondary feathers overlap produces maximum resistance during the down-stroke and minimum resistance on the up-stroke.
In gliding flight the wings are outspread and used as aerofoils, the bird sliding down a 'cushion' of air, losing height and gaining forward momentum. Sometimes upward thermal currents or intermittent gusts of wind may be used to gain height without wing movements; in seagulls and buzzards for example.
Generally, the fast-flying birds have a small wing area and a large span, with specially well-developed primaries, while the slower birds have shorter, wider wings with well-developed secondaries.
Estimates of speed vary from 160 km/h in swifts to 60 km/h in racing pigeons. The tail feathers help to stabilize the bird in flight and are particularly important in braking and landing.
In walking, the posture of the bird brings the centre of gravity of the bird below the joint of the femur and pelvis.
The detailed pattern of reproduction and parental care varies widely in different species but, in general, it follows the course outlined below.
Pairing. A sequence of behavioural activities, e.g. courtship display, leads to pair formation; a male and female bird pairing at least for the duration of the breeding season.
Nest building. One of the pair or both birds construct a nest which may be an elaborate structure woven from grass, leaves, feathers, etc., or little more than a hollow scraped in the ground.
Mating. Further display leads to mating. The male mounts the female, applies his reproductive openings to hers and passes sperm into her oviduct, thus enabling the eggs to be fertilized internally.
Egg laying. The fertilized egg is enclosed in a layer of albumen and a shell during its passage down the oviduct and is finally laid in the nest. Usually, one egg is laid each day and incubation does not begin until the full clutch has been laid.
Incubation. The female bird is usually responsible for incubation, keeping the eggs at a temperature approximating to her own by covering them with her body and pressing them against her brooding patches, i.e. areas devoid of feathers which allow direct contact between the skin and the eggshell. Incubation also reduces evaporation of water from the shell. At this temperature, the eggs develop and hatch in a week or two.
Development. The living cells in the egg divide to make the tissues and organs of the young birds. The yolk provides the food for this and the albumen is a source of both food and water. The eggshell and shell membranes are permeable, and oxygen diffuses into the air space, being absorbed by part of the network of capillaries which spread out over the yolk and over a special sac, the allantois, which has become attached to the air space. The blood carries the oxygen to the embryo. Carbon dioxide is eliminated by the reverse process through the eggshell. When the chicks are fully developed, they break out of the shell by using their beaks.
Parental care. The chicks of large, ground-nesting birds, e.g. pheasant, are covered with downy feathers and can run about soon after hatching. They peck at objects on the ground and soon learn to discriminate material suitable for food. They stay close to the hen, responding to her calls by taking cover or seeking her out according to the circumstances.
In most other species, the chicks hatch with few or no feathers, helpless and with closed eyelids. Having no feathers, they are very susceptible to heat loss and desiccation, and the parents brood them, covering the nest with the body and wings, so reducing evaporation and temperature fluctuations. Both parents will collect suitable food, often worms, caterpillars, insects and other materials equally rich in protein. The sound or sight of the parents approaching the nest causes the nestlings to stretch their necks and gape their beaks. The bright orange colour inside the beaks induces the parent to thrust the food it is carrying into the open beaks.
After a week or two, the young birds begin to climb out of the nest and sit in the bush or tree but the parents still find and feed them. When the primary and secondary feathers have developed, the fledglings begin short practice flights. This is one of the most dangerous periods of their lives since they can feed themselves to only a limited extent and cannot escape from predators such as cats and hawks. Some estimates suggest that only 25 per cent of the eggs laid in open nests of this kind reach the stage of fully independent birds.
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