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.

The chief muscle thus employed is the “great pectoral,” attached to the large keel (or ridge) on the breast-bone, and inserted into the “humerus,” or “arm-bone.” This “great pectoral” is generally the largest muscle in the bird’s body, and in fact often equals in bulk all the other muscles put together.

The wing is _opened out_ by straightening the elbow and the wrist-joints. The former process is effected by the contraction of the _triceps_; the latter chiefly by the action of the so-called “radial extensors,” and by the elasticity of the long “tensor,” or ligament, which comes from the shoulder muscles along the front border of the anterior wing-membrane, and is attached to the base of the thumb, at the front side of the wrist.

The wing is _folded_ by the bending of the elbow and the adduction of the wrist-joints. The elbow is bent principally by the contraction of the “biceps” and the “internal brachial,” the wrist-joint chiefly by the contraction of the “hand-adductor,” and of the “ulnar wrist-flexor.”

As already stated, the possession of feathers is one of the most characteristic features in a bird. These beautiful structures are modifications of the skin, just as are the scales of the feet and the claws of the toes. Feathers and hairs, scales and claws, are all produced out of, and are modifications of, the cells of the upper skin, or _epidermis_, and of the under, or true skin, or _cutis_. The feathers differ much in their minute construction in some birds; and all those of a bird are of course not of the same size and shape, but they have the following parts in common.

A feather consists of a quill, a shaft, barbs, and barbules: moreover, there may be a kind of accessory part, often in the shape of a downy tuft, close to the junction of the shaft and the quill. The shaft (_scapus_) or axis of every perfect feather (_penna_) is divided into the quill (_calamus_), the hollow cylinder (_d_), which is partly embedded in a sac of the skin, and the true shaft (_rachis_, _a_), which bears on each side the lateral processes called barbs (_rami_ or _radii_). The rachis and the barbs together are known as the vane (_vexillum_), and, in fact, form what is commonly known as the “feather” in contradistinction to the “quill.” The barbs (_c_, _c_, _c_, _c_) are narrow plates, or laminæ, “tapering to points at their free ends, and attached by their bases on each side of the rachis. The edges of these barbs are directed upwards and downwards, when the _vexillum_ of the feather is horizontal The interstices between the barbs are filled up by the _barbules_, pointed processes, which stand in the same relation to the barbs as the barbs do to the rachis. The barbules themselves may be laterally serrated and terminated by little hooks, which interlock with the hooks of the opposed barbules. In very many birds each quill bears two _vexilla_; the second, called the _aftershaft_ (_b_) (_hyporachis_), being attached on the under side of the first,”[133] close to the junction of the shaft with the quills.

In all the feathers of the Ratitæ, and in the case of all but the contour feathers in other birds, there are no barbules to the barbs. The pennæ are ordinarily arranged in definite patches, or areas on the bird, and the shape and size of these, and their relation to one another, differ in many birds.

The _aftershaft_ (_b_) is ordinarily a smaller _vexillum_, which is attached to the under side of the larger one at about the point where the rounded quill passes into the stem.

It is not necessary to notice these important characteristic structures more fully now, as they will have to be considered in explaining the distinctions between the great groups of birds, and we pass on to notice that the same kinds of birds are not found everywhere, but that they have, as groups, a remarkable geographical distribution.

In the following pages the distribution of birds is often alluded to, although it will naturally be impossible to discuss, within these limits, all the various phases of the study which the geographical distribution of the feathered tribes opens up to us. At the same time sufficient evidence will be given to show that birds are not scattered without order over the earth, but are more or less restricted to certain spots.

The six natural history or distributional provinces into which the world is ordinarily divided by modern naturalists were determined, first of all, from the study of the birds; and in fixing the boundaries of each division the wading birds and many swimming birds must be left out of the question, as they are creatures of such very extensive flight, and wander almost from pole to pole. A natural region, therefore, can be marked only by its resident forms of bird life, or at the most by the birds which breed within its limits; and the six regions alluded to provide us with many excellent reasons for believing that they possess well-defined physical boundaries. No Capercailzie, for instance, was ever found out of the _Palæarctic_[134] region, which comprises Europe and the greater part of Asia above the line of the Himalayas and the Yangtze-kiang River in China. This region is also characterised by a large number of Buntings, Warblers, Grouse, &c. In the _Nearctic_[135] region there is a certain similarity to the European and Siberian Avifauna, Grouse, Ptarmigan, Waxwings, Magpies, Ravens, &c., being commonly found throughout the two regions. North America possesses, however, several forms peculiar to itself, though it is by no means so rich in species as is the _Neotropical_[136] region, which commences south of a line drawn through Northern Mexico, and includes the whole of Central and Southern America. Within this large area are contained whole families of birds, such as Toucans, Mot-mots, the vast majority of the Humming-birds, Trogons, besides innumerable genera of Tanagers and other forms, so that this region is by far the richest in the world as regards bird life. The _Ethiopian_ region embraces all Africa below the Sahara Desert and Madagascar: Plaintain-eaters, &c., are characteristic of this region. The _Indian_ region skirts the Palæarctic, and includes the remainder of Asia below the Himalayas and the Yangtze-kiang; the Malayan Peninsula, the Sunda Islands, and the Philippines, belong to this region, which contains all the finest Pheasants in the world, the Impeyan Pheasant from the Himalayas, the Tragopans, and the Lobed Pheasant of Borneo being most beautiful creatures. Lastly, between the islands of Bali and Lombok passes a deep sea boundary called “Wallace’s line,” which divides the _Australian_ region from the Indian, and although these islands lie so close together, the great depth of the channel between them seems to mark them out as frontier lines of two ancient continents. Certain it is that the birds and animals on each side of Wallace’s line differ remarkably; and the Australian region, which includes all the Moluccas, New Guinea, and Oceania, in addition to the Australian continent and New Zealand, presents us with forms not found elsewhere, such as Birds of Paradise, Cassowaries, Lyre-birds, and a large variety of peculiar types. Many smaller divisions of the globe are now recognised, but the above are the main ones, which may occasionally be referred to in these pages.[137]

Many birds migrate, and the student of migration alone would find sufficient material there for the work of a lifetime; and it seems almost impossible to account for the instinct or other causes which bring birds regularly year by year to breed in the same haunts, and which drive them away at the same change of season. Why is it, for instance, that species of similar habits and form, and both visiting Europe in equal abundance, should occupy such different winter quarters? Yet the common Red-backed Shrike, or Butcher-bird (_Lanius collyrio_), when he is said to leave Europe, passes by the Nile Valley along the east coast of Africa down to the Cape, where he brings up a second brood of nestlings; while the Wood-Chat Shrike (_Lanius auriculatus_), a bird of about the same size and of precisely similar habits, proceeds down the Nile Valley and invades Abyssinia in the winter, and also occupies Senegambia, where a Red-backed Shrike has never been found yet by a naturalist. Nothing whatever is known by which route the bird gets to the Gambia: whether he follows the same one as his red-backed relation as far as Abyssinia, and then skirts the southern edge of the Sahara, or whether he reaches north-western Africa by a direct flight across the Great Desert. Many other such problems in the economy of our most familiar species are still awaiting further scientific research.