CHAPTER XV. THE PIGEON: A TYPICAL BIRD.

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:

(_a_) _Feathers._—In what direction do the free ends of the feathers point? Where are the longest feathers?

(_b_) _The head._—What is the general shape of the head? Notice the horny _bill_; is it blunt, pointed, or hooked? Does it bear teeth? Observe the _cere_, a whitish patch of swollen skin at each side of the base of the upper beak, and surrounding the _nostril_. Examine the large _eyes_, and notice the upper and lower eyelids and the transparent third eyelid. Find the _ear-opening_, a little below and behind the level of the eye, and hidden by the small feathers of the neck. Open the mouth and see the pointed _tongue_.

(_c_) _The neck._—In the feathered bird this appears short; it will be better seen after the removal of the feathers (contour feathers) which cover it.

(_d_) _The trunk._—What is the apparent shape of the trunk? Where is its heaviest part?

(_e_) _The wings._—Open out the wings fully and measure the distance from tip to tip; compare it with the length of the trunk. Which surface, upper or lower, of the expanded wing is rounded (convex) and which is hollowed (concave)? Put up an umbrella and, holding it by the handle, move it quickly (i) away from you, (ii) towards you. In which direction of motion does the air offer more resistance? Is it an advantage for the expanded wing to be convex above and concave below? Why? Feel the wing-bones through the skin, and notice that the wing-skeleton consists of three parts corresponding to the bones of your upper-arm, fore-arm, and hand (with the wrist) respectively; and that they are bent on each other in the form of a Z when the wings are folded. The large _quill-feathers_ attached to the segment which corresponds to the hand and wrist are called _primaries_; count them. Those attached to the fore-arm are called _secondaries_; count them. Find the tuft of smaller feathers which spring, at the front edge of the wing, from the thumb; this tuft is called the _thumb-wing_. The primaries and secondaries collectively may be called the rowing feathers. Notice the smaller feathers which, on both the upper and lower surface of the wing, cover the quills of the primaries and secondaries; these smaller feathers are called the _wing-coverts_.

(_f_) _The legs._—Feel the leg-bones through the skin, and make out the various segments. Is the whole of the hind-limb feathered? Which part is devoid of feathers? With what is this part covered? How many toes are there? Do any point backwards? Stretch out the legs and notice that the toes open. Bend the ankle-joint and notice that the toes close automatically. Of what use is this in perching?

(_g_) _The tail._—Examine the tail and notice the large quill-feathers attached to it; count them. As they are largely used for steering during flight they are sometimes called the steering feathers.

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:

(_a_) _The quill._—Is it hollow or solid? Try to bend it and notice its great strength. Observe the small hole at its base.

(_b_) _The shaft._—This is the prolongation of the quill, and carries the web or vane. Is the shaft hollow or solid? Notice the small tuft of down on the inner face of the feather, at the junction of the quill and shaft.

(_c_) _The vane._—Hold up the feather to the light and examine the vane with a lens. Notice that the vane consists of a number of laths which spring from the shaft; these are called _barbs_. Try to separate the barbs, and observe that they offer considerable resistance to the pull, as if they were somehow fastened together. Examine a barb with a strong lens and observe the finer branches, called _barbules_, which it bears. The barbules on one side of the barb carry little hooks, while those of the other side bear flanges, on which fit the hooks of barbules carried by the next barb below. The hooks and flanges cannot be seen without a microscope.

(_d_) _Other feathers._—Compare the _covert_ and _contour feathers_ with the quill feathers. Observe, in the contour feathers, the less perfect interlocking of the barbs of those parts which are covered by other feathers.

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:

(_a_) _The skull_, with the rounded brain-case, horny beak, and large eye-sockets. Notice, at the back of the skull, the _single_ knob, which fits into a hollow on the first vertebra.

(_b_) _The backbone._—Notice the long neck, the fusion of many of the vertebrae in the trunk-region, and, at the end of the tail, the “ploughshare bone.” This last supports the tail-quills.

(_c_) _The breast-bone_, or sternum, produced ventrally into a thin plate called the _keel_ (to which were attached the great muscles of the breast), and connected by ribs to the backbone.

(_d_) The position of the _shoulder joint_, and the socket for the bone of the upper-arm. Notice the =V=-shaped “merrythought.”

(_e_) _The bones of the fore-limb_ (wing).—Make out the parts belonging to the upper-arm, fore-arm, and wrist and hand, and notice that the bones of the wrist and hand are firmly fused together to give increased strength.

(f) _The large hip-bones._—Notice their forward extension and fusion with the joined vertebrae of this region, and the resulting increased firmness of this part.

(_g_) The bones of the _hind-limbs_.

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 184_a_), 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.

1. Why does a bird require a long neck, and a keel upon the breast bone? (1898)

2. Draw the bones of a bird’s wing, and mark the places of insertion of the largest quills. (1898)

3. What bones carry the primary and secondary quills of a bird’s wing, and the quills of the tail? Illustrate your answer by a drawing. (1901)

4. Point out the chief differences between the skulls of a quadruped and a bird. (1901)

5. Describe one of the large feathers of a bird’s wing. (1898)

6. Draw general plans of the fore-limbs of a mammal and a bird. Letter the principal bones, so as to show how they correspond in the two animals. (1901)

7. If the skull of a bird were placed before you, what features would enable you to recognise it with certainty? (1901)

8. How does the fact that a bird stands on two legs affect the skeleton of the trunk? (1897)

9. Point out the chief differences between the wing of a bird and that of a bat.

10. Explain how the barbs of a quill feather are attached to one another. Describe a feather at the time when it is forcing its way through the skin. (1905)

11. What peculiarities distinguish feathers which aid in flight from feathers which merely prevent loss of heat? (1906)

12. Describe the wing of a bird, both as to skeleton and as to external appearance. Compare the skeletal parts with those of the corresponding structures in a Mammal. (1908)

FOOTNOTE:

[16] Darwin, _The Origin of Species_ (Murray). Cheap edition, 1s.