ter, and two feet thick, but it is not very regular in its form. The strong, massive structure of the nest causes it to endure for many years, perhaps during the lives of the couple who build it, if they are not compelled to abandon it on account of danger or alarm. The necessity of building it so very strong will be more obvious when it is considered that the parent birds weigh from twenty-five to thirty pounds, the female being the larger of the two, as is common among birds of prey, exceeding the male in length by about a foot.

It is worthy of remark, that the same eagles vary their mode of building, when they cannot find a tree sufficiently large for their purpose, in the vicinity of their hawking grounds. In this case they make choice of a rocky pinnacle, forming the nest of the same materials in the upper portion, but dispensing with the rafters, which are there unnecessary, and placing the brushwood, moss, and leaves. directly over the stone; but the eggs are always deposited among chips of wood or sticks, and never upon softer materials *. Of this principle of variation in the mode of building nests, we shall have occasion, as we proceed, to give a considerable number of examples, which are highly interesting from the light they are calculated to throw on the faculties termed instinctive.

The bald eagle (Haliatus leucocephalus, SAVIGNY) seems to make a still more substantial nest. This bird is asserted by Wilson to be identical with the sea eagle (Haliætus albecilla, SAVIGNY); upon which point Latham is undecided; but Temminck rejects the opinion without hesitation, as he says he has seen more than fifty individuals of the sea eagle

* Vaillant, Oiseaux d'Afrique, i. 3...

food, and for this purpose the unconsumed portion of the yolk enters through the navel. The chick, indeed, which comes out of the shell before taking up all the yolk is certain to droop and die a few days after it is hatched. The help which I have occasionally tried to give to several of them towards their deliverance, has afforded me an opportunity of observing those which had begun to break their shells before this was accomplished; and I have opened many eggs much fractured, in each of which the chick had as yet much of the yolk not absorbed. Besides, some chicks have greater obstacles to overcome than others, since all shells are not of an equal thickness nor of an equal consistence; and I think it probable that the same inequality takes place in the lining membrane. The shells of the eggs of birds of various species are of a thickness proportional to the strength of the chick that is obliged to break through them. The canary-bird would never be able to break the shell it is enclosed in if that were as thick as the egg of a barn-door fowl; and the latter would crush all the eggs she might attempt to sit upon, if their shells were as thin as those of a canary-bird. The chick of a barn-door fowl, again, would in vain try to break its shell if it were as thick and hard as that of an ostrich; and even though an ostrich ready to be hatched is perhaps thrice as large as the common chick, it is not easy to conceive how the strokes of its bill can be strong enough to break a shell thicker than a china cup, and whose smoothness and gloss indicate that it is nearly as hard, sufficiently so indeed to form, as I have often seen, a solid drinkingcup. It is the practice in some countries to dip the eggs into warm water at the time they are expected to chip, on the supposition that the shell is thereby rendered more fragile and the labour of the chick

lightened. But even boiling water does not render the shell more fragile; and though the water should soften it, upon drying in the air it would become as hard as at first*." It is well remarked by Mr. Yarrel, that the shell is rendered more brittle by the process of hatching, during which the egg of the common fowl loses on an average eight grains a day, the moisture being partly evaporated and partly absorbed, and the lining membrane at the same time becoming partially separated †.

Eggs fractured by the included Chick.

Though the fracture of the shell usually extends round the whole circumference, yet the chick is sometimes contented with performing only about three-fourths of the task. When this has been done, the bill is not required to complete the separation, which is effected more easily and speedily by pushing with the whole mass of its body, using the feet as a lever. By thus continuing to push the body forwards, a movement which is frequently repeated, the chick gradually raises the upper portion of the shell, and at length tears all the fastenings; and if any part hold out, that becomes a sort of hinge which permits the lid, as we may call it, to be thrown on one Réaumur, as before. Zool. Journ. ii, 436.

side. When the upper portion is wholly detached, it is sometimes thrown to a considerable distance; but it often happens also that it is thrown into a very singular position, and is found placed within the other portion, as one cup may be set within another. This circumstance has been observed to arise from the chick, just escaped, having the upper portion of the shell immediately before him, and pushing it back with his feet into the under portion without any necessity or design. "A young duckling," says Réaumur, "which I observed just as it was striving to separate the two parts of the shell entirely, showed me that it had recourse, in order to this, to methods like those used by chickens and probably by all other birds for the same purpose. The duckling's shell was at most fractured in twothirds of its circumference, but the fracture being wide suffered me to see that the bill was under the right wing, while the little creature was lifting at the forepart of the shell on the side where it did not resist, because there all its fastenings were broken, and by these means it forced the shell to break on the side where it was yet entire."

"When the chick," Réaumur continues," has at length been able to turn up or heave off sufficiently the forepart of the shell to procure itself a door, it

stretches out its feeble legs, which are yet insufficient to carry it. Being then entirely or almost out of the shell, it draws its head from under the wing where it had hitherto been thrust, stretches out its neck, and directs it forward, but is not strong enough for several minutes to raise it. Upon seeing for the first time a chick in this condition we are led to infer that its strength is exhausted, and that it is ready to expire; but in most cases it recruits rapidly, all its organs gather strength, and in a very short time it appears quite another creature. After having dragged itself on its legs for a little while, it becomes able to stand on them, to lift up its neck, to bend it in various directions, and finally, to hold up its head. The feathers are at this period only fine down; and as they are wet with the fluid matter of the egg, the chick appears almost naked. By the multitude of their branches these down feathers look like so many minute shrubs; yet when those branches are wet and sticking to each other, they take up very little room, but as they dry they become disentangled and separated. The branchlets, plumules, or beards of each feather are at first enclosed in a membranous tube, by which they are pressed and kept close together; but as soon as this dries it splits asunder, an effect which is also aided by the elastic springs of the beards themselves causing them to recede and spread about. When this is accomplished, each feather extends over a considerable space, and when they all become dry and straight, the chick appears completely clothed in a warm vestment of soft down*.'

It would not be according to the usual course of nature unless the greater number of eggs sat upon proved fertile; but circumstances, many of them unintelligible to us, occur during hatching, which render * Oiseaux Domestiques, as before.