IMPERIAL EAGLE.
THE CLASS AVES.—THE BIRDS.
CHAPTER I.
INTRODUCTION—WING-STRUCTURE AND FEATHERS—DISTRIBUTION.
Introduction—Distinctive Characters of the Class Aves—Power of Flight—The Wing—Its Structure—The Six Zoo-geographical Regions of the Earth—Birds peculiar to these Regions.
THE study of birds is almost an instinct in an Englishman: from peasant to noble, an innate affection for the feathered songsters seems to prevail; so that whether it be in a stately aviary or in a little cage outside a cottage door, birds are found to be the constant companions of man throughout the length and breadth of the land. And it is possible that no other country in the world possesses such a number of birds, in proportion to its size, as does Great Britain. Any one travelling on the continent of Europe cannot fail to notice how few birds meet his eye; and although they may be there, and may be found by a little searching for, they do not form such a prominent feature of a walk as they do in England or Scotland. Even the toiler in large towns has but to get a little way into the nearest fields to hear the cheering song of the Skylark or the Thrush, or to be amused with the bustling and active habits of the Starling, or those of the more sedate and cautious Rook. It is certain that a study of the habits of birds will always repay the student, who may see in the feathered favourites which are around him many a little bright example to be followed, if he read the lesson aright. Birds teach us many things—perseverance, fidelity, parental affection, thrift, cleanliness, and many other domestic virtues, which are to be seen carried out in their life in the greatest perfection. In the following pages the birds will be passed in review, and the habits of some of the most striking and important forms will be detailed. But, although every species and genus cannot be noticed here, it is necessary to assure the student that in every country—even in England, where so much has been done for Ornithology—he will find an ample reward in the study of all birds; and that even the commonest species cannot be neglected, for there is always something new to learn and to record in their life-history. To quote Dr. A. E. Brehm, one of the most accomplished observers of nature, who, carefully trained by his father, a true naturalist also, has studied the feathered tribes in many climes:—
“He who is only half at home with nature on this earth of ours will be able approximately to appreciate the feelings with which the naturalist wanders and travels from place to place: wherever he may be he finds friendly forms. For years he has silently watched the interior economy and household arrangements of animated nature, and yet he has not seen all; and on this account he is never in want of employment. Every bird is a personal friend: the old ones he knows almost as well as he knows himself, and the new ones must be studied. How much more is there yet to observe! Rich as he may be in experiences, every fresh ramble brings him fresh mental treasure. The relations existing between him and the bird become each day more and more intimate; he knows the lives and habits of each: when each arrives, or takes its departure: where is its abode: how it is made: when it is occupied by a happy troop of nestlings: when deserted. The naturalist knows his friends by their notes, flight, and bearing. In his eye the bird never ceases to exist: alive or dead it is always interesting in his eyes, for in either case the bird is associated with a poesy of feeling in creative nature which he would put into words. Every new bird raises his spirits a step higher; every fresh discovery is a step onward in the knowledge of the ways and means of all things. He is indebted to his friends for many a happy hour; their lives are a pattern worthy of imitation.”[132]
Here, then, we may ask—What is a bird? How can a bird be told from all other Vertebrate animals? The chief character which distinguishes the class of birds is undoubtedly the clothing of the body with feathers. Other characters they also possess, but not exclusively. For instance, they have the power of flight developed in the greatest degree: but there are some birds, such as the Apteryx, the Ostrich, and the Cassowary, which cannot fly at all; while, on the other hand, there are flying mammals, such as Bats, Flying Squirrels, and there are flying reptiles, which can progress through the air by means of flight. Again, birds lay the eggs from which they produce their young; but so do many reptiles and fish: so that this cannot be considered a prerogative of the class of birds. Their bill is hard and sheathed in horn; but so is that of the Duck-billed Platypus (page 231), an animal belonging to the Monotreme Mammals; and Turtles also have beaks. Most, but not all, birds build nests; and in this they stand almost alone among the higher animals; but nest-building propensities are developed in many of the Mammalia—in the Lemurs and Mice, for instance—while it cannot be assigned as a habit peculiar to birds, as the wonderful nests made by some fish conclusively prove.
All birds, whether they fly or not, are clothed with feathers, and this distinguishes the class Aves in the existing state of nature. The majority are specially adapted for flight: and as this is undoubtedly the most vigorous form of locomotion, the greatest muscular efforts being required to raise and sustain a body above the ground and to propel it rapidly through the air, a large development of muscular energy is necessary. The great strain on the circulation of the blood is met by a heart not only as complete as in the Mammalia, but with stronger and a peculiar valvular mechanism for propelling that fluid vigorously through the body. Moreover, in addition to their lungs, birds possess a singular provision of air-receptacles within the body, and these are connected with a series of cavities, also filled with air, which occupy the interior of most of the bones. These cavities serve not only to give lightness to the bird’s body, but they also assist the lungs in aërating the blood, so that birds may be said to enjoy a double respiration. As birds exceed mammals in the activity of their breathing and circulatory system, so also they possess a higher degree of animal heat, their temperature ranging from 106° to 112° Fahr. This high temperature, which exceeds that of the Mammalia by from 8° to 14°, is maintained by its admirable feather-clothing, which, being a non-conductor, effectually serves to guard against any sudden variations of temperature in the air to which its body is exposed, during its rapid and extensive flights, as well as tempering the usual radiation from the body.
As one might expect in the fore-limb of a creature specially organised for suspension in, and progression through, the air, it is found that the muscles, as well as the bones and joints, of the bird’s wing become much modified as compared with the corresponding parts of other animals. With all our scientific knowledge and mechanical contrivances, no one has yet succeeded in constructing a flying machine. It is a significant fact that Nature has not only long ago solved this problem, but that she has done so in several ways. The flight of an insect, of a bird, of a bat, is equally perfect in its way: but in each case the result is attained by very different modifications in the skeletal and muscular apparatus. The principal resistance that a flying animal has to work against is its weight: that is, the force of gravity which, proportionately to its mass, tends to draw it down vertically towards the earth; hence the muscles which are largest and strongest in a bird are those which pull down the wing against the air, thereby raising the body and overcoming its weight.