BIRDS constitute the fourth class of vertebrates. They may be defined as warm-blooded creatures, with a double circulation and a covering of feathers, which produce their young from eggs. This definition can be elaborated, but it contains all the essentials and there are no exceptions to it.
Birds breathe more actively and completely than any other vertebrates. They absorb air not only by the lungs but also by some of the bones. All birds, except the Apteryx of New Zealand, possess a number of cavities which form air receptacles. Consequently the aeration of the blood is
more complete in their case, and this, together with the perfect construction of their heart and high muscular activity, gives them a higher average temperature than any other living creatures.
HOW A BIRD’S BONES HELP IT TO FLY
THE skeleton of birds is very compact and at the same time exceedingly light. This is partly due to an unusual quantity of phosphate of lime and also in many bones to the absence of marrow and its replacement by air. This lightening of the skeleton is one great aid to flight; another is the development of a keel-like breast-bone for the attachment of the powerful flying muscles.
There is a great difference in the flying powers of various birds, and many may be termed specialists in some particular branch. Compare for instance the soaring of larks, the swift flight of swallows, the sturdy power of pigeons, wild ducks, and swans; the mastery of the wind displayed by kestrels, the breath-taking speed and dash of falcons and the glorious strength of condors and eagles on the wing.
Feathers form a unique growth whose evolution is obscure. All birds moult once a year (some twice), generally after the breeding season, and many assume a special mating plumage. Feathers are useful for retaining heat; they assist greatly in flight and play a great part in courting. Though the theory of protective colouring has often been overworked, there is little doubt that during nesting many female birds are protected by their unobtrusive plumage.
The brains of birds are relatively larger than those of reptiles. Many birds are forced to rely on the alertness of their senses for their sole protection, hence the corresponding increase in brain-power. As regards their physical senses, the eyes are always well developed, and in no bird are they ever rudimentary or absent. Birds have a third eyelid which can be drawn over the surface of the eye in the manner of a curtain, shutting out excessive light.
Most birds have no external ear by which sounds can be transmitted to the internal ear, but in the case of the owls there is an arrangement of feathers which is a forerunner of the external ear of mammals. Taste must always be absent in the majority of birds owing to the horniness of the tongue in most species, whilst their sense of touch cannot be much better. In no living birds are teeth ever present, although in
some extinct forms they are known to have existed. The gizzard is a relic of a reptilian ancestry, Being by virtue of their wings independent of time and place, birds are able to avoid winter and secure the best food all the year round. Whereas some mammals take refuge in a winter sleep when cold weather cuts off the supply of food, the birds simply travel to another region in search of it. The habit of migration is indeed of great service to the race.
It may be noted that when birds migrate northwards it is to find a suitable temperature for breeding, but when they move southwards it is to secure food and warmth. Out of about four hundred recognised British birds, only about one-third remain in this country all the year round; the rest are migrants and summer visitors.
Apart from man, birds are perhaps the only creatures with a sense of beauty. Some species appear to have a natural capacity for selecting certain bright colours. This is well shown by the Bower Birds of New Guinea, small birds related to starlings, which, in the breeding season, not only make elaborate bowers or arbours of sticks in which the males pay court to the hens, but ornament these bowers with bright shells, feathers, etc., and even in some cases change any objects, such as flowers, which have become withered. Curiously enough some of the falcons are known to decorate their nests by the addition of fresh, green branches.