51. HABITS AND STRUCTURE OF THE PIGEON.

1. General observations upon the dovecote pigeon.—Watch a group of pigeons. What is the shape of the body? With what is the body covered? What is the colour of the feathers? Does the bird walk or hop? How many walking limbs has it? Has it any other means of moving from place to place, in addition to walking? How many wings has it? Are the wings anterior or posterior (p. 217) to the legs? Watch a pigeon preening its feathers; why does it apply its bill so frequently to its tail? Can the bird bend its neck easily in all directions? On what do pigeons feed? How do they pick up their food? Do they chew the food? What is the voice of the pigeon like? At what time of the year do pigeons moult? How many eggs does the hen-pigeon lay? What is the colour of the eggs? What are newly-hatched pigeons like? How do the parent birds feed them?

Try to find a wood-pigeon’s nest. What is its shape? What is it made of? Is the top open or closed-in?

2. The external characters.—Closely examine a dead pigeon in respect of the following features:

3. The feathers.—Examine the arrangement of the feathers. The short feathers which clothe the body generally are called contour feathers because they determine the contour of the unplucked bird; remove one or two and place them aside. The feathers which cover the bases of the large quills are called coverts—wing-coverts or tail-coverts according to their position. Remove one or two for future examination. Examine the arrangement of the quill-feathers of the wings, and notice how the vane or web of one partially overlaps the next. The vane is supported by a shaft. Are the two sides of the vane of equal width? Is the narrower side directed forwards or backwards? Pull out a quill-feather from each wing and compare them. How could you recognise, if you did not know, whether any particular feather had come from the right or from the left wing? Pull out a quill-feather from the tail; are the sides of the vane of equal width?

4. Examination of various feathers.—Examine in detail a quill-feather from the wing. Make out:

Pluck the pigeon, and notice the hair-like structures left in the skin. They are called filoplumes. Pull one out and examine it with a lens; it bears a few barbs at its upper end, but these do not interlock.

Examine a nestling-pigeon, and notice the down-feathers which clothe it temporarily. Each is at first covered at the base with a horny sheath. The barbs do not interlock. The down-feathers are pushed off by the growth of the permanent feathers.

5. The plucked pigeon.—Observe that feathers do not grow on all parts of the body, but are confined to definite feather-tracts, which can be recognised by the scars left by the quills. Notice especially the sockets of the large wing-and tail-quills. Make out the oil-gland, a small knob just above the tip of the true tail. It furnishes a lubricating fluid used in preening the feathers. Examine more closely the different regions of the body—the neck, the joints of the limbs, etc., which were disguised by the feathers. The filoplumes have already been seen. Feel the great muscles of the breast, and the edge of the sternum or breast-bone in the middle line. Just in front of this feel the soft crop, with the grain which is probably present in it.

Before boiling the bird in order to separate and examine the bones, open the crop and inspect its contents. Cut away the enormous muscles of the breast (why are they so large?) and open the body-cavity behind and remove the internal organs, being careful not to break any bones.

6. The skeleton.—Boil the bird until the flesh can be easily removed. A small nail-brush will be found useful in cleaning the bones. Keep as many of these in contact as possible, and make out and examine the following parts:

7. Pneumatic bones.—Examine the bone of the upper-arm of the pigeon and notice, just below the head of the bone, a small hole which leads into the interior. Break the bone across; is it hollow or solid? Does the inside contain marrow? How does it differ from the corresponding bone of a rabbit?

8. Different breeds.—Examine various breeds of domestic pigeons, and compare them with the common variety, as regards colour, shape, method of flight, extent of feathering of the hind-limbs, etc.

Birds.—It is probable that no class of animals has been more studied than that of the birds; and the reasons for this are not difficult to find. The wonderful power of flight—rivalled only by the insects—and the perfect adaptation of structure to this power; the habits, always interesting and in many cases showing a curious parallelism to human institutions; the beautiful voices of song birds; the grace of movement; the mysteries of migration; and, it may be added, the remarkable story of the origin of birds which has been revealed by modern zoology,—form a combination of characters, at once familiar and elusive, which has greatly stimulated human sympathy and imagination. It is well to begin the study of birds by first taking one familiar species and examining it somewhat closely, and afterwards comparing and contrasting other members of the class. Such a convenient type for study is found in the dovecote pigeon.

The pigeon.—The body of the common dovecote pigeon (Fig. 178) is ovoid in shape, tapering gently at the neck and appearing, in the living animal, to pass insensibly into the head. The fore-limbs are modified to form a pair of wings, which are set on a little above and in front of the centre of gravity of the body. Except during flight the weight of the body is entirely supported by the hind-limbs or legs, which are fixed almost exactly below the centre of gravity of the body, i.e. in the best imaginable position. Each foot bears four toes, the first of which is directed backwards. Each toe is armed with a claw.

The body generally is clothed with feathers, remarkable structures which are as characteristic of birds as hairs are (p. 220) of mammals. The great majority of the feathers are small, overlapping and plate-like, with their free ends pointing backwards; and they form a light, warm, and smooth covering which is admirably adapted to the animal’s needs. Certain large feathers, carried by the wings and tail, are used in flight. The legs are feathered to the ankle-joint, but the feet are covered only with scales. The general colour of the dovecote pigeon is a slaty blue.

The head is rounded, and terminates in front in a horny bill, which does not bear teeth. At the base of the upper beak there is, on each side, a whitish patch of swollen skin called the cere, which surrounds the opening of the nostril. The eyes are large and round; in addition to upper and lower eyelids, each is provided with a transparent third eyelid, which can be flicked rapidly across the eyeball. Birds generally have very powerful sight and depend more upon this sense than upon any other. There are no external ears, but when the feathers are separated, a little below and behind the eyes, a pair of apertures leading to the internal ears may be seen.

Habits.—The pigeon lives upon grain, which it picks up by means of its horny bill. The length and flexibility of the neck are a great help in feeding. The food is swallowed immediately and passed into a large bag called the crop, which is really a dilatation of the lower part of the gullet. Here it is macerated for some time before it passes onward to the true stomach. The hinder part of the stomach is called the gizzard; it has very thick walls and a hard horny lining. In the gizzard the food is ground up by the aid of small stones which the pigeon swallows for the purpose. The stomach is followed by a much-coiled intestine, in which the process of digestion is completed.

The pigeon differs from many birds in walking instead of hopping. When perching, it bends its legs at the ankles, an action which automatically closes the toes in such a manner that the perch is grasped behind by the first toe and in front by the second, third, and fourth. The weight of the body is sufficient to maintain a tight clasp upon the perch, so that the bird is able to sleep comfortably in this position.

The hen-pigeon lays two white eggs, which are sat upon by the parents for fourteen days and thereby kept at a temperature of about 40 degrees Centigrade. The young birds then break through the shell and are hatched. As the chicks are at first quite helpless and unable to feed themselves, the parents supply them with a milky fluid secreted by their crops. This is sometimes called “pigeon’s milk.” Newly hatched pigeons are covered with fine feathers called down, which are pushed off by the development of the permanent feathers beneath them. The nests of wood-pigeons (Fig. 180) are usually built in trees; they are somewhat rough structures, composed of twigs, and open at the top.

The wings.—A bird’s wings are full of interest, from whatever point of view they are considered. Even now, the exact manner in which they and the tail are used to bring about the many different movements of flight is not fully understood; but a general idea may be obtained by an examination of the bones and feathers and the careful observation of flying birds.

The bones of the fore-limb and the neighbouring regions show a remarkable modification of the primitive plan. The bones of the upper arm OA, (Fig. 181) and fore-arm (Rd., Ul.,) are arranged much like the corresponding bones of the rabbit; but those of the wrist and hand are unlike anything we have yet examined. The fourth and fifth fingers have entirely disappeared; and the first (thumb), second, and third, though they can be still recognised (Figs. 181, F, F, F and 184a), have become consolidated with what remains of the wrist (HW, MH) to form a firm mass which is in no danger of “giving” during the powerful down-stroke of the wing. The head of the bone of the upper arm fits into a socket (gl.cv., Fig. 182) at the junction of the shoulder-blade (scp.) and another and stouter bone (cor.) which runs back and is attached to the breast-bone or sternum (st.). The sternum in its turn is connected by means of ribs with the spinal column, which is strengthened here by the fusion of some of its bones. The ventral part of the sternum is produced into a large plate called the keel (car., Fig. 182), which gives attachment to the great muscles of the breast, used in the movements of the wings. So great is the development of these muscles in flying birds that in the pigeon they have a weight equal to one-fifth the total weight of the body. The three segments of the wing, corresponding to the upper-arm, fore-arm, and wrist and hand respectively are, in the position of rest, bent upon each other like the letter Z. The large quill feathers of the wing, fixed along the hinder border of the limb, are in two series (Fig. 179). Those attached to the fore-arm are called secondaries; while those attached to the wrist and hand are known as primaries. A smaller tuft of feathers borne by the thumb is called the bastard-wing or thumb-wing. The smaller feathers covering the bases of the wing-quills are called wing-coverts.

The hind-limbs and tail.—The hind-limbs, or legs, of birds are the sole support of the body except during flight, and a relatively great weight is thus thrown upon them. To provide for this, the skeleton of the hip region is exceptionally strong. Not only are the bones of the spine here welded together into one solid mass, but the hip-bones extend forward much further than is the case in quadrupeds, and are firmly attached to the fused vertebrae. Each hip-bone possesses a socket (actb., Fig. 182) into which fits the head of the bone of the upper leg. Another fusion of bones is to be noticed at the end of the tail, resulting in the ploughshare-bone (Fig. 182, pyg. st.) which supports the large steering feathers of the tail. The quills of these feathers are covered at their bases by the tail-coverts.

Just above the end of the tail of a plucked pigeon is to be seen a small conical body, the oil-gland, from which the bird obtains a fluid which it applies to the feathers during the preening process.

The feathers.—Feathers do not grow upon all parts of a bird’s body, but are restricted to certain definite areas or “feather tracts” which, however, differ in arrangement in different species.

One of the large feathers of the wing of the pigeon is illustrated in Fig. 183, A. It consists of a hollow, horny quill (cal.), prolonged into a solid shaft (rch.) which supports the web or vane. The outer face of the feather is slightly convex and smooth; while the inner surface is somewhat concave and rough, and carries a little tuft of down at the junction of quill and shaft. The growing feather is nourished by a conical projection of the deeper part of the skin, which fits into a small hole (inf. umb.) at the base of the quill. It is easily shown that the vane of the feather is composed, on each side, of a large number of parallel laths which spring from the shaft. These laths are called barbs. Considerable resistance is felt when an attempt is made to pull the barbs apart, but the manner of their connection cannot be clearly seen without the aid of the microscope. A fairly low magnifying power, however, shows (Fig. 184) that each barb bears extremely delicate threads—barbules—on each side, arranged on the barb much as the barbs are arranged on the shaft. The barbules on one side of each barb (the side furthest from the quill) are seen to carry tiny hooks; while the barbules of the other side of the barb are furnished with flanges. The hooked barbules of one barb cross the flanged barbules of the next and interlock with them; so that relatively great force is required to pull the barbs apart and destroy the continuity of the web. The quill-feathers of the tail, and the coverts, have webs of similar structure.

The contour-feathers are used for protection and warmth rather than for flight, and hence have less-perfectly interlocking barbs. The hair-like structures seen on the skin of the plucked bird are extremely simple feathers called filoplumes (Fig. 183, B). Each consists of a long, slender axis, bearing at the end a few barbs which do not interlock.

The down-feathers (Fig. 183, C), which form the temporary covering of nestling pigeons, also bear barbs which do not interlock.

At regular intervals, either once or twice a year, birds moult, that is, shed most or all of their feathers and grow new ones. The moulting usually takes place gradually and symmetrically: a flight feather from each wing, for example, being dropped at the same time.

Flight.—The outstretched wing of a pigeon has a relatively great area; and is markedly convex on the upper surface and concave on the lower, resembling in this respect an open umbrella. The great difference in the resistance which the inside and outside of an open umbrella respectively oppose to the wind is familiar to everyone, and illustrates the advantage of the dome-shape of the outstretched wing. The force which the great breast-muscles put into the down-stroke of the wing is enormous in comparison with the weight of the body, and is sufficient to push the bird upwards and forwards in the air. The gently tapering shape and smooth surface of the feathered bird diminish the resistance of the air. When the wing is raised again for the next stroke, the quill-feathers are turned a little edgewise, so that the air slips between them; just as an oarsman “feathers” his oar to lessen the resistance to the blade in the return-stroke. The direction of the wing-stroke can be altered in accordance with the direction of air currents, and the fan of tail-feathers is capable of being opened or closed, raised or lowered, and turned at various angles to act as a rudder.

But the method of flight which perhaps most of all excites the observer’s admiration is soaring, in which the bird seems to remain almost passive, with outstretched wings and spread tail, and mounts automatically in a spiral course. Exactly how the soaring is performed, only the birds themselves know; but it must be similar in essence to the arrangement of the sails of a yacht so as to select that component of the force of the breeze which will drive the boat in the required direction, even though this be almost “in the teeth of the wind.” Such spiral soaring may be seen to perfection, among common British birds, in the skylark (p. 316). A breeze is necessary for soaring.

In hovering, the motion of the wings is extremely rapid, and the bird remains poised in one place. The kestrel (p. 330) derives its common name of “windhover” from its habit of using this method of flight when looking for food.

The pigeon’s air supply.—We know that a man breathes more quickly when he is taking active exercise than at ordinary times; and we might, from this and similar observations, expect to find the most perfect breathing organs in animals which lead the most active lives. Of all vertebrate animals, birds probably perform the most work in proportion to their size; and it is not surprising, therefore, to learn that they have special facilities for quickly replacing fouled air by fresh.

The lungs are comparatively small, but they are not the only organs of respiration (p. 242); for the windpipe opens also into several air-sacs, which supply nearly all parts of the body with air, and even communicate with the interior of certain bones. The bone of the pigeon’s upper-arm, for example, is hollow and contains air. By means of the system of air-sacs, the air in the lungs is completely renewed at each respiratory act, and thus the waste carbon-dioxide in the blood can be exchanged for fresh oxygen much more completely than it is in mammals. One result of this is that the blood of birds is much warmer than that of mammals. The air-sacs are also an assistance to flight by rendering the body more buoyant.

Different breeds of pigeons.—The various breeds of domestic pigeons furnish one of the best examples of the changes in structure which breeders can bring about, after several generations, by careful crossing. In spite of the great differences between, say, the tumbler, with its habit of turning head-over-heels in the air; the pouter, with its exaggerated crop; the Jacobin, with its neck-feathers reversed; the fantail, with its large number of erect tail-feathers; and others,—there can be no doubt that all these varieties have been artificially produced from the rock-pigeon; and it is curious to observe occasional reversions to the characters of the ancestral stock. “The rock-pigeon is of a slaty blue, with white loins.... The tail has a terminal dark bar, with the outer feathers externally edged at the base with white. The wings have two black bars. Some semi-domestic breeds, and some truly wild breeds, have, besides the two black bars, the wings chequered with black. These several marks do not occur together in any other species of the whole family. Now, in every one of the domestic breeds, taking thoroughly well-bred birds, all the above marks, even to the white edging of the outer tail-feathers, sometimes concur perfectly developed. Moreover, when two birds belonging to two or more distinct breeds are crossed, none of which are blue or have any of the above-specified marks, the mongrel offspring are very apt suddenly to acquire these characters.”[16] This is a striking instance of the tendency exhibited by living things to revert to ancestral characters, even though these latter may have lain dormant for hundreds of generations.

EXERCISES ON CHAPTER XV.