Cassell's book of birds; vol. 1

by Alfred Edmund Brehm

The muscular activity of every animal is intimately dependent upon the efficiency of its breathing apparatus, upon the freedom with which the vital element finds admission to the blood which it is destined to renovate, and upon which it confers those qualities so inseparably connected with the elimination of increased temperature, and the vigour of muscular action. In this respect, as we shall see immediately, the feathered races surpass all living creatures, with the exception, perhaps, of the members of the insect creation.

The lungs of a bird are not suspended, like those of a quadruped, within a circumscribed chest or thoracic cavity, in such a manner as to become inflated by each inspiration; they are rather to be described as soft, porous, and highly vascular organs, through which the air passes as through the interior of a sponge. The movements of the chest, upon which depend the inspiration and expiration of the atmospheric fluid, may be compared to those of a bellows continually employed in taking in and expelling the surrounding element by a mechanism represented in the accompanying figure (Fig. 1). The framework of the chest, consisting of the ribs and of the breast-bone, is so put together that at each inspiration it can be raised, as shown in the drawing, from the position d to the position h, thus materially enlarging the thoracic chamber, just as the upper board of an ordinary bellows is raised for the purpose of taking in the air; but, in this case, the surrounding element, instead of entering through a valve-defended orifice, rushes down the windpipe, and through the immovable, sponge-like lungs, permeating the wide passages with which they are perforated, and not only filling the entire thorax, but penetrating into the interior of the very bones, which are left marrowless for its reception.

The mechanism whereby expiration is effected is equally simple; just as, when the upper board of the bellows is depressed, the air is forced out through the nozzle, so, by the return of the breast-bone to its former position, the inspired air is again forced to pass through the lungs and make its escape by means of the windpipe. By this process it is obvious that the vital element—the oxygen of the atmosphere—being admitted to every part of the system, the blood is vitalised to the greatest possible extent, its temperature is raised until the heat of the body of a bird is far greater than that of an ordinary quadruped, and its vitality is proportionately exalted. Consequently, as the blood circulates through the system, it carries with it heat and life in superabundance; the energies of the entire system are roused to the uttermost; the fibres of every muscle quiver with intense life, like a steam-engine working under high pressure, thus enabling the falcon to cleave the skies with the velocity of a falling thunderbolt, and not only qualifying the swallow for its rapid flight, but enabling it to achieve its wonderful migrations.

This admission of air into every part of the system serves not only to fan the vital flame, and rouse the energies of the bird to an extraordinary degree of tension; it likewise assists in giving buoyancy to its movements, bearing it upward, as the gas does a balloon; for it is evident that the air received into the body being raised to a temperature corresponding to the heat of its blood, the specific gravity of the bird is proportionately diminished, and it rises into the air almost without an effort, and even hovers in the sky with scarcely a perceptible movement of its wings.

A knowledge of the mechanism of their mode of respiration will likewise enable us to explain another remarkable feature in the history of the feathered tribes, namely, their power of song. Who that has listened to the prolonged warblings of a linnet, the flood of melody poured forth from the little throat of the canary, the "lengthened sweetness long drawn out" which almost pains the enraptured ear as we listen to the song of the nightingale, but has wondered how such tiny birds can ever find sufficient breath for the utterance of such long-sustained, such interminable notes? What would our prima-donnas at the opera give for but the tithe of the capacity of these favoured little songsters? No human breast could ever hold sufficient breath for such performances. We now see, however, that the vocal organs of a bird are exactly adapted to the nature of their music. Their whole body is a bellows, as large in proportion to their size as the bellows of an organ is in relation to the pipes into which it has to pour the sound. The little bird is, in fact, a living harmonium—its singing apparatus is not situated at the top of its throat, but is implanted in the inferior termination of its windpipe; and just as the tongue of the harmonium is thrown into vibration by the issuing current of air caused by pressure upon the bellows, so are the vocal chords of the feathered songster rendered sonorous as the air passes over them. In proportion to the capacity of the bellows must be the duration of the note, and we have already seen that the air-cells of the bird are capable of furnishing a supply not easily exhausted. There is, however, this remarkable difference between the two instruments: the tongue of each key of the harmonium can give utterance but to one sound—one never-varying tone—while the corresponding part of the bird, rendered more or less tense by muscles provided for the purpose, contains within itself a whole gamut, and there is not a note in the scale that is not instantly at the command of the inimitable little musician. In the perching birds, among which are found by far the most accomplished singers, five pairs of muscles are connected with this exquisitely-contrived apparatus, and are so disposed as to influence both the diameter and the length of the air-passages. In the parrots three pairs are met with; some of the swimming birds have two, while others have only one; and in a few—as the king of the vultures and the condor—vocal muscles are quite wanting.

Seeing that the temperature of birds is raised so much above the usual standard by the arrangements described above, some clothing is requisite, adequate to retain the vital heat. Another indispensable provision is therefore met with in the Feathers with which all birds are so warmly clad. Indeed, so peculiar is the texture of these admirable fabrics, that no better distinctive appellation could be devised for the entire class than that of the "feathered tribes," by which they are frequently designated. A feather realises in its structure more qualities than imagination could have conceived it possible to combine—lightness, thickness, warmth, durability, elasticity, softness, strength, and beauty. It is one of the master-works of creation. Whoever has examined a feather under the microscope will testify to the incomparable perfection of the contrivance. Every feather is a mechanical wonder. If we look at the quill-portion, or barrel, we find it possessed of attributes not easily brought together—strength and lightness. If we cast our eye upon the upper part of the stem, we see a material made for the purpose, which is used in no other class of animals, and in no other part of birds—tough, light, pliant, elastic—the pith. This is also a substance sui generis; it is neither bone, flesh, membrane, nor horn.

Fig. 2.—WING OF A BIRD PARTIALLY STRIPPED OF FEATHERS, TO SHOW THE INSERTIONS OF THE QUILLS.

a, the Arm; d, the Fore-arm; g, the Thumb; c, the Secondary Quills, implanted into the Fore-arm; f, the Primary Quills, implanted into that portion of the Wing which represents the hand; e, the Spurious or Bastard Quills, derived from the Thumb.