"The bird that hath been limed, with trembling wings misdoubteth every bush."-SHAKSPERE.

of a summer's sun; and arrive in the midst of our own fine weather.

There are also some which never remain in temperate climates, but merely pass through them on their annual migrations. The time of the arrival and departure of these travellers is generally fixed definitely for each species, and in many instances may be calculated upon to a day.

945. Another fact not less curious in the history of birds is the power by which they direct their course in unknown countries, and distinguish at immense distances the immediate vicinity of their nests. Swallows furnish a remarkable instance of this. These little birds make very long journeys at the time of migration; and yet, by a singular instinct, they know, in the following spring, how to find again the places where they had formerly established themselves, and always return to them. This fact has been ascertained by attaching to the feet of several swallows small silken threads to establish their identity. They build their first nest near that in which they were born; the chimney-swallow makes its nest every year above that of the preceding year, and the house-swallow establishes itself in that which it had quitted the autumn before. Indeed, it has been known for the same couples to repair to their old nests, almost without taking the trouble to repair them, for eighteen years consecutively. Swallows show on other occasions, also, a singular power of directing themselves towards a particular place from which they are at a considerable distance. If a hen that is sitting is carried to a distance, confined in a cage, and liberty afterwards given to her, she first rises to a considerable height, as if to reconnoitre the country, and then flies in a straight line to the spot where she has left her brood.

946. Why do birds when flying in flocks settle themselves into a triangular body?

Because the position so assumed is the one best fitted to cut the air with the greatest ease and rapidity.

In this mode of flight, the leading bird, which divides the air in advance of the others, has the most arduous post; and in order to obtain relief, the bird occupying this place goes to the rear after a certain time, and is succeeded by another.

947. Why do birds which take long flights usually fly high?

It is supposed that high flight is less fatiguing, owing to a

"The wanderers of heaven

Each to his home retire, save those that love

To take their pastime in the troubled air,

Or skimming flutter round the dimply pool."-THOMSON.

certain density of the atmosphere, which is best suited to the weight and action of the wings of birds, and which cannot be found nearer the earth.

Another reason also may be, that the upper regions to which birds thus ascend are exempt from the momentary gusts and squalls to which the lower atmospheric strata are subject.

948. Eagles are heavy birds, even for their powerful wings, and yet they are high flyers, although their abodes are at great elevations in the mountains. Wild geese and herons take the sky when they set out on long journeys. Rooks may be seen to adjust the height of their daily excursions from the rookeries to the distance at which the pasture upon which they feed lies; and the swallow tribe wheel about far more rapidly and gracefully when they fly high before rain, than when they skim the pools in fine weather. When birds are in long and swift flight, they acquire a momentum in proportion to their velocity, and the difference between their specific gravity and that of the air. In consequence of this momentum, they continue their progressive motion with much less effort, which is of itself sufficient to explain why they prefer flying high.

949. Why are birds which winter out of England supplied with a cover of black down under their feathers?

Because black is a colour capable of retaining the greatest amount of heat, and in this instance the object is, to keep in the heat arising from the heart and the circulation of the blood.

950. It is likewise remarkable that this provision is not found in larger birds-for which there is also a reason. Small birds are much more exposed to the cold than large ones, forasmuch as they present, in proportion to their bulk, a much larger surface to the air. It is necessary, therefore, that small birds should be more warmly clad than larger ones; and this appears to be the expedient by which that exigency is provided for.

951. Why are the birds which are brought alive to England from distant regions generally hard-billed?

Because hard-billed birds subsist on seeds which are easily carried on board ship; while the soft-billed birds, which are supported by worms or insects, or as a substitute, fresh raw meat, can obtain neither in long voyages.

"Meanwhile the tepid caves, and pens, and shoares,
Their brood as numerous hatch, for the egg that soon
Bursting with kindly rupture forth disclos'd

Their callow young."-MILTON.

952. Why have some species of birds extremely soft bills?

Because the class of birds thus furnished have to seize their prey quickly, as they often catch it while it is on the wing; though they do not fly after it, rapidity of motion both in darting at the food, and in opening and shutting both with firmness and sharpness at the tip, are the requirements of such a bill; and lightness in its general structure is essential to quick motion. These bills are, in very many species, so thin and weak that they are not able to break the coat of a vegetable seed.

A shell impervious to air protects the germ of life within, until from two to twenty eggs have accumulated, and then, although laid at different intervals, the incubation commences, and the young birds are hatched at the same time.

954. Why do the eggs of birds contain two distinct substances, the white and the yolk?

The white, or albumen, of the egg, supplies nourishment to the

"The man who can see without pleasure a hen gather her chickens under her wing, or the train of ducklings follow their parent into the pond, is like him who has no music in his soul."-KNOX.

chick whilst it is in the shell; the yolk is embraced in the body of the chick when excluded from the shell, and a duct leads from the membrane enclosing this mass of nutriment into the first intestine. Thus the chick is nourished, not only whilst enclosed in the shell, but also during its first feeble existence.

955. When we hold an egg steadily, and chip it at the upper part, we find the yolk close to the shell, and on the upper surface a pale vesicle, 3, which contains the

embryo chick. When the hen sits, the heat of her

body developes the action of the living principle in the embryo, and on the second and third day a little zone of blood-vessels, 4, 4, 4, appears; these vessels run towards the embryo, and carry nourishment to it; and day by day we may watch its sensible growth. From the delicacy of this action, we may perceive how necessary it is that the embryo at an early period should be close to the breast of the hen, and not at the cold bottom of the nest. We shall now see how it is accomplished:-The yolk is a globe of nutritious matter, and the little vesicle with the embryo is involved in the surrounding membrane, and consequently is at the surface of the globe. If this globe had the axis of its revolution in the centre, it would not move with the change of the position of the egg. But the axis being below the centre, it must turn round with every change in the position of the egg, whether the globe be heavier or lighter than the surrounding white: were it heavier, it would revolve so as to bring the embryo to the lower part of the shell -were it lighter, to the upper part of the shell. It is lighter, and the matter stands thus:-The yolk is, as it were, anchored at two points, and the attachments being below the centre, and the yolk being lighter than the surrounding white, it revolves like a buoy, and the vesicle containing the embryo is thus kept always uppermost.*

956. Why has the young chick a hard scaly substance upon its beak?

This curious provision is designed to assist the young bird in breaking the shell, at a time when its bill is too soft for that purpose.

The manner in which the young chick breaks the egg is one of

* Paley's "Natural Theology."

"The yolke of the egge cannot be without the whyte, nor the whyte without the yolke; no more maye the clergy and the lordes be one without another."-FROISSART'S CHRONICLES.

the most wonderful operations of instinct. The instrument which it employs is a small protuberance on its upper mandible, called the bill-scale, which has no other use, and accordingly drops off soon after the bird is hatched.

957. This bill-scale is provided with a sharp cutting edge, the use of which is to cut through the membrane which lines the shell. Were it not for this, the shell

would break, while the membrane would stretch, and the chicken would still remain a prisoner.

The egg-shell is formed by a solidified deposit from the blood-vessels of the egg-duct of the parent bird; but it is permeable to air, which is necessary to the life of the embryo. The shell is lined by two membranes: the one external and rough, so that it adheres to the shell; the other exceedingly smooth, so that it allows of the rotation of the contained parts. And, at one extremity of the egg, there is a little sack filled with air containing an unusual proportion of oxygen, which is employed in giving, vitality to the awakening germ.

958. Why can a recently hatched bird exist for many hours without food?

Because a portion of the yolk of the egg yet remains in its body, and by that it continues to be nourished.

959. The wisdom of this provision is obvious: had the first-born bird needea immediate food, it must either have been starved, or the mother, whilst providing for it, must have deserted her other eggs, and thus destroyed the rest of her family; but, under this arrangement, the mother may, without injury to her first-born, continue to sit on her nest till all her little ones are hatched, which sometimes takes more than a day.