Читать книгу Our Domestic Birds: Elementary Lessons in Aviculture - John H. Robinson - Страница 5

Table of Contents

Feathers. The feathers of a bird are the most highly developed form of protective covering in animals, serving other important functions in addition to the primary one. Compared with the hair of a mammal or the scale of a fish or of a reptile, a typical soft feather from the body of a bird is a very complex structure, partaking of the characters of both scales and hair. The fact that birds have scales and hair as well as feathers shows their relation to these other forms of animal covering. This is best observed on a fowl. The legs of a fowl are normally smooth, with scales on the front of the shank and on the upper surfaces of the toes. In feather-legged fowls the feathers appear first along the outer sides of the shanks and toes. As the number of feathers is increased they grow longer and more feathers appear, until in the most heavily feathered specimens the soft skin is covered and the scales are almost hidden.

The face of a fowl is normally almost bare, the skin being a bright red like the comb and wattles; but at a distance of a few feet we can usually see some very small, fine feathers on it, and if we examine closely we see in addition still finer growths—hairs. Among the body feathers of a fowl, too, are quite long hairs. These are most easily observed after a bird is plucked. They do not come out with the feathers, and are removed by singeing.

Structure of feathers. The smallest feather that to the naked eye appears as something more complex than a hair, looks like a little bunch of fuzzy filaments. This is called down.

In the next higher form of feather a small round quill appears with filaments protruding from it like the hairs in an artist's brush. Such a feather is called a stub feather, or simply a stub. The best place to find these is on the outside of the shank of a fowl with scantily feathered legs.

The first form of the complete feather is best observed either on the head of a fowl or at the hock joint. The feathers in these places are very small, yet complete. The round quill is lengthened into a shaft. Extending from each side of this shaft is a single row of filaments, called barbs, the edges of which, interlocked with little hooks, form the web of the feather. On other parts of the body of the bird the feathers are larger, but the general structure is always the same. The size and special structure of the feather are always adjusted to suit the part on which it grows or the service which it has to perform.

As the first function of the feathers is to keep the bird dry and warm, the body feathers are all soft as compared with the large stiff feathers of the wings and tail; yet as we look at the feathers on different parts of the body of a bird we notice differences in their structure, and also notice that the structure of a feather is not always the same throughout its length. On the exposed parts of the feathers of the neck, back, wings, and breast the web is perfect and the feathers overlap so closely that they present a smooth surface. Under the surface, especially next the skin, the barbs are not smoothly joined, but are fluffy. Thus the same feathers which present a hard, smooth surface to the weather provide a soft, warm garment next the skin. Under the wings and on the underside of the body the feathers are quite fluffy throughout their whole length.

Arrangement of the feathers. As you look at a living bird the feathers appear to grow on all parts of the body. When the feathers are removed from the bird you see that while the skin is nearly all rough, with the little elevations where the feathers were removed, there are quite large areas where the skin is perfectly smooth, showing that no feathers grew there. These places are bare because feathers on them would interfere with the movements of the bird. The feathers on adjacent parts give the smooth areas sufficient protection.

Decorative feather forms. The natural decorative forms of plumage are found mostly in male birds and consist of extraordinary developments of the plumage of the neck and back, where the male birds of some species always have feathers differing in form from the feathers on the same parts of the female. When a feather appendage not common to a species is developed on some varieties, as the crest and beard on fowls and the ruff on pigeons, both sexes have it. The most interesting feather decorations will be described particularly in the chapters on the species on which they occur.

Fig. 1. Brown Leghorn chick (one day old)

Color in feathers. While colors in the plumage are distributed very differently in different species of birds, often making combinations peculiar to a species, there is in all the same wonderful formation of patterns, that depends for its effect in a section upon some overlapping feathers being marked alike and others having a different marking; and for the effect in a single feather, upon adjacent barbs being now alike, now different, in the distribution of the pigment in them. The best common example of a pattern covering a series of feathers is found on the wing of a Mallard Duck or of a Rouen Duck. Interesting examples of the formation of patterns on a single feather may be found in the plumage of barred, laced, and penciled fowls, and also in the lacings on the body feathers of the females of the varieties of ducks mentioned. Perhaps the most interesting illustrations of this kind, however, are to be seen on the plain feathers of the guinea and the gorgeous tail of the peacock.

The pigment which colors the plumage may be found in soluble form in the quills of immature colored feathers. It is not conspicuous unless it is quite dark. In black fowls it is often so abundant that a part remains in the skin when the feathers are removed. After the pigment is deposited in the web of the feather the color is fast. Water does not affect it, but it fades a little with age and exposure. New plumage usually contains a great deal of oil, a condition which is most conspicuous in white birds, to whose plumage the oil gives a creamy tint. In colored birds the presence of a large amount of oil in feathers is desirable because it gives greater brilliance to the plumage.

Fig. 2. White Leghorn chicks (ten days old)

Growth and molting of feathers. The first covering of a young bird is down. The young of birds which nest on the ground have the down covering when hatched; others acquire it in a few days. In small land birds which feather quickly, as Leghorn and Hamburg chicks, the largest wing feathers may have started to grow before the chick leaves the egg. In most kinds of poultry, however, the young show no signs of feathers for some days. The down is gradually replaced by small feathers, and these by larger feathers as the bird grows. As feathers in all stages of growth are found on the young bird at the same time, it is not known whether all feathers are molted the same number of times. In cases where some feathers were marked and watched, or where the colors changed with the changing feathers, it appeared that after the down three sets of feathers were grown in succession, the third and last making the adult plumage of the bird. This coat remains until the following summer or fall, when it is molted and replaced by a new one.

Flight. The habit of flying is objectionable in domestic birds because it makes them more difficult to control. It has no direct use except in pigeons kept for flying. There is, however, a very important connection between development for flying and the value of birds for the table. The muscles of the wings furnish the greater part of the edible meat of most birds. The most desirable birds for food purposes are those which have the wing muscles well developed, yet not quite strong enough to enable them to fly easily. In such birds the breast meat remains comparatively soft through life, while in birds that fly well it becomes hard in a very short time. That is why the breast meat of the pigeon is relatively tougher in an old bird than the breast meat of a fowl or turkey.

The balance between capacity for flight and neglect to use it, which is desired in birds grown for the table, is secured by giving them opportunity to exercise their wings moderately but not for progressive practice in flying, which would soon enable them to fly easily over the fences used to confine them. To regulate such exercise the perches for birds that roost are made low, or in an ascending series in which each perch after the first is reached from the one below it, while fences are made so much higher than the distance the bird is accustomed to fly that the failures of its first efforts to go over them discourage it. Ducks and geese, which do not roost, flap their wings a great deal, and if they have room often exercise them by half running and half flying along the ground.

Mechanism of the wing. In its structure and in the muscular power that moves it, the wing of a bird is a wonderful piece of mechanism. A bird in flying strikes the air with its wings so rapidly that the movements cannot be accurately counted. The heron, which is a slow-flying bird, makes from one hundred twenty to one hundred fifty downward strokes of its wings a minute. As each downward stroke must be preceded by an upward stroke, this means that the wings make from two hundred forty to three hundred separate movements a minute. In such swift-flying birds as the pigeon the movements of the wings can be distinguished but cannot be counted. The fastest movements of the wings are not made by the swiftest fliers. In order to fly at all some land birds with comparatively small wings have to move them so fast that the movements make a blur and a whirring noise. The partridge is an illustration of a bird of this class.

If the supporting surface of the wing of a bird were made of skin, like the web of the foot of a swimming bird, it would be necessary to fold the wing for each upward stroke. It is here that the structure of feathers adapts itself to the rapid action required for movement in the air. The wing is not one surface but a series of narrow surfaces lapping in such a manner that they unite to form one broad surface when the downward stroke is made, and with the upward stroke are separated so that the air passes between them. Greater power in the downward stroke and less resistance in the upward stroke are also secured by the curvature of the wing. The under side is concave, the upper side convex. Thus in the downward stroke the wing gathers the air under it and so increases the pressure, while in the upward stroke it scatters the air and reduces the pressure.

If the wing were equally rigid throughout, the movement of the bird would be mostly upward. The bird in flying moves forward because the front of the wing is rigid and the tips of the feathers, which are directed backward, are flexible. So the air compressed by the wing in the downward stroke escapes backward, and in doing so propels the bird forward. The principle is the same that is applied in the screw propeller of a boat or an airship, except that the wing vibrates while the propeller revolves.

The most important function of the tail in flight is to balance the bird. It is of some assistance in steering, but a bird steers its course mostly by manipulation of the wings.

Scratching. With the exception of the aquatic birds and the ostrich, all the species of poultry belong to the group called by naturalists Rasores or Scratchers. Birds of this class have legs of moderate length and very strong, with toes terminating in a stout claw. Normally they have three toes upon which the foot rests when they are standing on a flat surface, and a fourth toe, like a thumb, which assists the other toes to grasp a perch. Some individual birds and some races of birds have the fourth or hind toe double. The leg of a bird is so constructed that when it is bent as the bird sits on a narrow support the toes contract and grasp the support and hold it without any effort on the part of the bird. Thus the bird is as secure in its position on a limb when asleep as if wide awake and looking out for itself.

In proportion to their ability to scratch, birds are able to find seeds and insects concealed among dead or living vegetation on the surface of the ground, and also to dig below the surface. Scratching capacity is most highly developed in the fowl. Compared to it the other land birds are very feeble scratchers, and do little damage by scratching if free to roam about. For ages the scratching propensity of fowls was regarded as a vice in them, but since people began to give special attention to poultry they have learned that fowls are much more contented and thrifty in confinement if their food is given them in a litter of leaves, straw, or shavings, in which they must scratch for it, and have also found that to some extent fowls may be used to cultivate crops while destroying insects and weeds among them.

Swimming. Capacity for swimming has an economic value in domestic birds because it adapts those possessing it to places which land birds rarely frequent. It will be shown when the different kinds of aquatic birds are described that each has its special place and use in domestication.

The swimming faculty in these birds is of further interest because of its relation to the development of the body plumage. If a land bird is placed in the water, the feathers are quickly saturated, the water penetrating to the skin. A duck or other swimming bird will remain in the water for hours without the water penetrating the feathers. This is commonly supposed to be due to the presence of a large amount of oil in the feathers, but the difference in the oiliness of the feathers of fowls and of ducks is not great enough to account for the difference in resistance to the penetration of water. The peculiar quality of the plumage of swimming birds is its density. If you take up a fowl and examine the plumage you will find that it is easy to part the feathers so that the skin can be seen. It may be done with the fingers, or even by blowing gently among the feathers with the mouth. Now try to separate the feathers of a duck so that the skin will be visible. You find it much harder, because the feathers are so thick and soft and at the same time so elastic. The familiar phrase "like water from a duck's back" is not especially appropriate. The feathers on the back of most birds are a very effective protection against rain. The feathers all over a duck are such poor conductors of water that it is hard to remove them by scalding. The structure of the plumage of swimming birds adds to their buoyancy in the water. They do not have to exert themselves to remain on the surface, but float like cork.

Foods and mode of digestion. All kinds of poultry and most of our common wild birds are omnivorous eaters, but the proportion of different foods usually taken is not the same in different kinds of birds. Some eat mostly grains, some mostly animal foods. Some can subsist entirely on grass if they can get it in a tender state; others eat very little grass. The scratching birds like a diet of about equal parts of grain, leaves, and insects. Pigeons and canaries live almost entirely on grains and seeds, but like a little green stuff occasionally.

Domestic birds which produce many eggs require special supplies of food containing lime to make the shells. Until within a few years it was universally believed—and it is still commonly supposed—that birds needed grit to take the place of the teeth nature did not give them, and assist in the grinding of the food in the gizzard. Many close observers now reject this idea because they find that birds supplied with digestible mineral foods do not eat those that are not digestible. A bird does not need teeth to grind its food, because it is softened in the crop and the gastric juice acts upon it before the grinding process begins.

Peculiarities of birds' eggs. The only animal foodstuff produced in a natural package, easily preserved and handled, is the egg. In the vegetable world we have a great many such things—fruits, seeds, roots, nuts, with coverings of various textures to protect the contents from the air. In all of these the material stored up is either for the nourishment of the seeds in the first stages of growth as plants, or for the nourishment of a new or special growth. An egg is the seed of an animal. All animals produce eggs, but in mammals the new life originating from the egg goes through the embryonic stages within the body of the parent, while in insects, fishes, reptiles, and birds the egg is laid by the creature producing it before the embryo begins to develop.

In mammals the embryo grows as a part of the body of the parent, the substances which build it up coming from the parent form as they are needed. In birds a tiny germ—the true egg—is put, with all the material needed for its development as an embryo, in a sealed package, which may be taken thousands of miles away from the parent, and, after lying dormant for weeks, may begin to grow as soon as the proper conditions of temperature are applied. The food value of the germ of an egg is inappreciable. We use the egg to get the material stored up in it for the young bird which would come from the germ.

Development of the egg. The method of the formation of an egg is very interesting. It is the same in all birds, but is most conveniently studied in fowls. If a laying hen is killed and the body is opened so that the internal organs can all be seen, one of the most conspicuous of these is a large, convoluted duct having its outlet at the vent. In this duct, which is called the oviduct, are eggs in various stages of formation. At its upper extremity, attached to the backbone, is a bunch of globular yellow substances which are at once identified as yolks of eggs in all sizes. The organ to which these are attached is the ovary. The smallest yolks are so small that they cannot be seen without a powerful microscope. These yolks are not germs, but as they grow the germ forms on one side of each yolk, where it appears as a small white spot.

When a yolk is full-grown it drops into the funnel-shaped mouth of the oviduct. Here it is inclosed in a membranous covering, called the chalazæ, and receives a coating of thick albumen. The function of the chalazæ is to keep the yolk suspended in the center of the egg. It does not merely inclose the yolk, but, twisted into cords, extends from either end and is attached to the outer membrane at the end of the egg.

After leaving the funnel the egg passes into a narrow part of the oviduct, called the isthmus, where it receives the membranous coverings which are found just inside the shell. From the isthmus it goes into the lowest part of the oviduct—the uterus. Here the shell is formed, and at the same time a thin albumen enters through the pores of the shell and the shell membranes and dilutes the thick albumen first deposited. After this process is completed the egg may be retained in the oviduct for some time. It is, however, usually laid within a few hours.

Rate and amount of egg production. In the wild state a bird, if not molested after it begins laying, produces a number of eggs varying in different kinds, according to the number of young that can be cared for, and then incubates them. If its first eggs are removed or destroyed, the bird lays more, usually changing the location of its nest. In domestication the eggs of most kinds of birds are removed from the nests daily as laid, and the birds lay many more eggs before they stop to incubate than they do in the wild state.

It is, and has been for ages, the common opinion that the wild birds and poultry, when first domesticated, were capable of laying only a small number of eggs each season, and that laying capacity has been enormously increased in domestication; but the oldest reports that we have of the amount of egg production indicate that the laying capacity of fowls was as great centuries ago as it is at the present time. Recent observations on wild birds in captivity show that even birds which pair and usually lay only a few eggs each season have a laying capacity at least equal to the ordinary production of hens. Quails in captivity have been known to lay about one hundred eggs in a season, and an English sparrow from which the eggs were taken as laid produced over sixty.

The constitutional capacity to produce ovules is now known to be far greater than the power of any bird to supply the material for the nourishment of germs through the embryonic stage. The principal factors in large egg production are abundance of food and great capacity for digesting and assimilating it.

Incubation. A bird before beginning to lay makes a nest. Some birds build very elaborate and curious nests; others merely put together a few sticks, or hollow out a little place on the ground. In birds that pair, the male and female work together to build the nest. Even in polygamous domestic birds like the fowl and the duck, a male will often make a nest for the females of his family and coax them to it as a cock pigeon does his mate.

If the birds are left to themselves and the eggs are not molested, an aërial bird will usually lay a number of eggs equal to the number of young the parents can feed as long as they require this attention, while a terrestrial or aquatic bird will usually lay as many eggs as she can cover. The desired number of eggs having been laid, the process of incubation by the parents begins.

The incubation of their eggs by birds is one of the most remarkable things in nature. We say that "instinct" leads birds to build their nests and to keep their eggs warm for a period varying from two weeks for small birds, to six weeks for the ostrich; but "instinct" is only a term to describe the apparently intelligent actions of the lower animals, which we say have not intelligence enough to know the reasons for the things that they do.

Fig. 3. Sitting hen

The mother of a young mammal knows that it came from herself, and she can see that it is like her and others of her kind. It at once seeks her care and responds to her attentions. The egg which a bird lays is as lifeless—to all appearances—as the stones which it often so closely resembles. Only after many days or weeks of tiresomely close attention does it produce a creature which can respond to the care lavished upon it. The birds incubating eggs not only give them the most unremitting attention, but those that fill their nests with eggs before beginning to incubate methodically turn the eggs and change their position in the nest, this being necessary because otherwise the eggs at the center of the nest would get too much heat and those at the outside would not get enough. A bird appears to know that if she begins to sit before she has finished laying, some of the eggs would be spoiled or would hatch before the others; and, as noted above, aërial birds seem to know better than to hatch more young than they can rear. But no bird seems to have any idea of the time required to hatch its eggs, or to notice the lapse of time, or to care whether the eggs upon which it sits are of its own kind or of some other kind, or to know whether the young when hatched are like or unlike itself. If eggs fail to hatch, domestic birds will, as a rule, remain on the nest until the eggs are taken away or until sheer exhaustion compels them to abandon the hopeless task. In domestication, however, those birds which continue laying most freely when their eggs are removed as laid, tend to lose the habit of incubation. Turkeys and geese will often begin to incubate after having laid about the number of eggs that they could cover. Many fowls will do the same, but most fowls lay for several months before attempting to incubate, and in many races not more than two or three per cent of the hens ever incubate.

Fig. 4. Fresh egg[1]

Fig. 5. Infertile egg (after twenty-four hours' incubation)

Fig. 6. Fertile egg (after twenty-four hours' incubation)

Fig. 7. Embryo (after seventy-two hours' incubation)

Fig. 8. Embryo (after seven days' incubation)