ZOOLOGY.

BIRDS.

GENERAL ECONOMY.

Why are birds said to constitute, as it were, an isolated class of beings?

Because they are distinguished, by certain characters, from all other animals: their classification does not pass into any other, and cannot, therefore, be consistently introduced into the supposed chain or gradation of natural bodies. With regard to form, all birds coincide in having two feet, two wings, abill, (either partly or entirely horny,) and a body covered with feathers. Blumenbach.

Birds are extremelyimportant creatures in the economy of Nature in general; although their immediate utility to mankind is infinitely less than that of mammifera. They destroy innumerable insects; and the thoughtless extirpation of some birds, supposed to be noxious, as sparrows, crows, &c, in many districts, has generally given rise to an infinitely more prejudicial multiplication of vermin. Other birds destroy larger animals, as field-mice, snakes, frogs, lizards, or consume carrion. Many extirpate weeds. On the other hand, they assist the increase and propagation of animals as well as plants. For instance, it is known that wild-ducks, in their emigrations, carry impreg-nated spawn into remote ponds, &c, and thus stock them with fish.* Many birds swallow seeds, which are subsequently expelled whole, and thus extensively dispersed: as the doves of Banda, with the nutmeg. The excrement of sea-birds manures bare cliffs and coasts, so as to render them capable of producing useful plants. Many species of falcons may be taught for the chase, as well as the cormorant for takingfish. Many birds, together with their eggs, fat, &c, serve for food; the entire skins of sea-birds for the clothing of many Northern nations; the feathers for stuffing beds, for writing, for various and often costly ornaments; in which respect, also, they form an important article of trade among many savage people, particularly the islanders of the Pacific Ocean. - Blumenbach.

Why are birds usually classed according to the forms of their bills and feet?

Because those parts are connected with their mode of life, food, &c, and influence their total habit very materially. - Blumenbach.

Why have birds little power of suction?

Because of the narrowness and rigidity of their tongue; as may be seen when they drink, having to hold up their heads, and depend upon the weight of the water for transmitting it into the craw. - Rennie. Why are birds said to be 'poised' in the air? Because the centre of gravity of their bodies is always below the insertion of their wings, to prevent them falling on their backs, but near that point on which the body is, during flight, as it were, suspended.

The positions assumed by the head and feet are frequently calculated to accomplish these ends, and give to the wings every assistance in continuing the progressive motion. The tail also is of great use, in regulating the rise and fall of birds, and even their lateral movements. - Fleming.

Why do birds fly?

Because they have the largest bones of all animals, in proportion to their weight; and their bones are more hollow than those of animals that do not fly. Air-vessels also enable them to blow out the hollow parts of their bodies, when they wish to make their descent slower, rise more swiftly, or float in the air. The muscles that move the wings of birds downwards, in many instances, are a sixth part of the weight of the whole body; whereas, those of a man are not in proportion one-hundredth part so large. Why does flying differ from leaping?

Because flying is the continued suspension, and progress of the whole body, in the air, by the action of the wings. In leaping, the body is equally suspended in the air, but the suspension is only moinentary. In flying, on the contrary, the body remains in the air, and acquires a progressive motion by repeated strokes of the wings on the surrounding fluid. Fleming.

In swimming on the surface of the water, the legs of birds are exclusively employed; but when motion is accomplished beneath the surface, the wings are then chiefly in exercise.

Why are birds covered with feathers ?

Because, by this addition to the non-conducting appendices of the skin, birds are enabled to preserve the heat, generated in their bodies, from being readily transmitted to the surrounding air, and carried off by its motions and diminished temperature. Fleming.

PART IV.

7*

Why are the strongest feathers of birds in the pinions and tail?

Because the pinion-feathers may form, when the wing is expanded, as it were, broad fans, by which the bird is enabled to raise itself in the air and fly; whilst its tail feathers direct its course. - Blumenbach.

Why are some few birds unfit for flight?
Because they have scarcely any pinion feathers,
Blumenbach.

Why do birds moult?

Because they may be prepared for winter; this change being analogous to the casting of hair in quadrupeds. During summer, the feathers of birds are exposed to many accidents. Not a few spontaneously fall: some of them are torn off during their amorous quarrels; others are broken or damaged; whilst, in many species, they are pulled from their bodies to line their nests. Hence, their summer dress becomes thin and suitable. Previous to winter, however, and immediately after incubation and rearing of the young is finished, the old feathers are pushed off in succession by the new ones, and thus the greater part of the plumage of the bird is renewed. Fleming.

Why do certain birds appear to change colour in passing from one place to another?

Because colour is the gift of light. Thus, the indigo-bird appears at one time of a rich sky-blue, at another of a vivid verdigris green. When the angle of incidence in the rays of light reflected from the plumage of the bird is acute, the colour is green, when obtuse, blue. The colour of the head being of a very deep blue, is not affected by any change of position.

Why does the plumage of many Northern birds change from a dark colour to a pure white, on the approach of winter?

Because such change increases the warmth of the birds; the white colour reflecting heat most readily, and suffering it to escape but slowly by radiation.

The completeness of this provision for the changes of the season, is beautifully illustrated by the deep colours of the summer dress of birds, being exchanged for the white colour of winter, with a rapidity and extent proportional to the changes of the seasons. During a mild autumn, the shifting of the dark for the light-coloured dress proceeds at a very slow pace; and when the winter also continues mild, the white dress is never fully assumed. In some species, as the black guillemot, the white winter dress is never acquired in this climate, although its ash-coloured plumage intimates a tendency to the change. In the climate of Greenland, on the other hand,

d. the change is complete, and the plumage is of a snowy whiteness.* Fleming.

Why is it erroneous to imagine, that Northern birds, by thus becoming white, are better enabled to escape their foes, from their resemblance to the colour of the snow?

Because, if their white dress concealed them from their enemies, these last, by being deprived of their ordinary food, would be in danger of starvation and death. Neither is this variation of colour confined to weak or defenceless animals. Beasts and birds of prey are likewise subject to the change. Hence, if it yielded protection to some, it would enable others to prey with greater certainty of success on their defenceless neighbours; and would not thus be consistent with the wisdom and benevolence of the Creator. The popular opinion on the subject must, therefore, be relinquished; especially as the change of colour, from dark to white, does not vary, however different the habits, or even stations, of the animals may be, in which it takes place. - Fleming.