CHAPTER VIII.

The Wings of Nestling Birds.

"The blue eggs in the Robin's nest Will soon have wings, and beak, and breast, And flutter and fly away."--_Longfellow._

The wing of the unhatched bird--Of the coots and water-hen--The Hoatzin's wings--The wing of Archæopteryx--Moulting--The nestling game-birds and ducks--Teaching the young to fly.

At first sight it may seem a little strange to introduce nestlings into a book devoted to birds in flight. But there are aspects of the wing of nestling birds which must, indeed, be borne in mind when considering the wing of the adult.

It was pointed out, in Chapter I, that the wing of the adult had but three fingers and two wrist-bones. This condition represents the last stage in the evolution of the Avian wing. The wing of the nestling gives a clue to an earlier stage in its history. But we can get even further back than this. For if we examine the wing of an unhatched bird, we shall be able to get still nearer to the birth, and growth of the wing out of a reptilian fore-limb. Here as many as six wrist-bones may be found. And the "palm-bones," which in the adult are welded together, are here quite separate. This stage, then, carries us back towards the ancestral, reptilian, fore-limb used for walking, or perhaps for climbing. And there is another sign of this earlier, reptilian, period to be found in such a wing. At the tip of the thumb and first-finger, in unhatched ducks, game-birds, and water-hens, for example, you will find a small claw. By hatching-time the claw of the first finger will have disappeared, but it is still retained in the case of the duck and the water-hen. In the adults of all three you will rarely find more than the claw of the thumb: and this now serves no useful purpose whatever.

Indeed, there seem to be only two tribes which have any use for wing-claws during nestling life. One of these is represented by the gallinules, that is to say, the coots, and water-hens, and their kind. You may test this whenever you have the good fortune to capture a young water-hen. Place him outside the nest, and especially if it happens to be a little raised, you will see him make his way back, using feet, wing-claws, and beak. His wings, it will be noticed, at this stage are used as fore-legs. The other tribe is represented by that strange bird the hoatzin of the Amazon. Here the two claws are really large, and they play a quite important part in his early life.

For the young hoatzin is hatched in a nursery--a crude nest of sticks--placed on the boughs of a tree overhanging the water. As soon as hatched he begins to climb about the branches. Should he fall, by some mischance, into the water, he promptly swims to the bank; and by the aid of his long first finger, and wing-claws, and his huge feet, soon climbs back. But the most wonderful part of his story is yet to come.

So long as these youngsters can only scramble about they are in constant jeopardy. A wing-surface at least big enough to break the force of a fall is an urgent necessity. And so the growth of the quill-feathers is, so to speak, pushed forward with all possible speed. But if all the feathers grew at the same rate, there would speedily come a time when the outermost feathers would make the claw at the end of the finger useless, while the wing-surface, as a whole, would be insufficient. To obviate this difficulty, the development of the outermost feathers is held in abeyance till the inner feathers of the hand, and the outermost of the fore-arm, have grown big enough to suffice to break the force of the fall. As soon as this stage is arrived at, the outermost quills, whose growth has been held in abeyance, rapidly develop; the finger decreases in length, and its claw disappears, while that of the thumb soon follows suit. And thus it comes about that the hand, in the nestling, is relatively much longer than in the adult. But in its mid-period it may be taken to represent the adult stage of the wing of the ancient Archæopteryx. This bird could have been but a poor flier, and probably during the time it was moulting its quills it was absolutely flightless, so that it needed a permanent finger-tip, and claw, beyond the margin of its wing-surface.

This matter of "moulting," by the way, needs, at least, passing comment. All birds renew their plumage at least once: the body plumage often twice in the year. The old feathers fall out, and their places are taken by new ones. But their growth is slow. In geese and ducks, and some other birds, the wing-quills are moulted all at once, so that flight, for a week or two, is impossible. But they can escape from their enemies while thus at a disadvantage, by taking to the water. In all other birds the quills are moulted, and renewed, in pairs: so that at no time are they left flightless.

But this by the way. Let us revert, for a moment, to the hoatzin's wing. The appearance of the outermost quills of the hand, it will be remembered, is delayed till the inner feathers have grown long enough to "flutter," at least, for a short distance, then the growth of the complete series proceeds apace. This has been called an "Adaptation" to enable these youngsters, active from the moment they leave the egg, to move about in comparative safety. But it is more than this. It is a survival of an ancient order of things which takes us back to the first known birds.

This is certainly a very remarkable feature, but it gains an added interest from the fact that it has a parallel in the history of the development of the wing in the game-birds. If you look carefully at the downy chicks of the pheasant, or even at barn-door fowls, you will remark that the wing-quills develop with surprising rapidity: so that they have feathered wings while the rest of the body is still down-covered. This enables them the more easily to escape prowling foxes and other enemies. In young ducks exactly the opposite condition obtains, the body is fully feathered long before the feathers of the wings appear. And this because they do not need to fly when danger threatens, but take to the water instead. But to return to the chicks of the pheasant. The wing of the chick develops at a very rapid rate. Within a few hours after hatching, the first traces of the coming flight feathers can be seen, and presently a large wing is covering each side of the tiny body. At this stage many often die. The wings, which can then be examined at leisure, reveal an extremely interesting condition. For they repeat the features which obtain in the wing of the nestling hoatzin: inasmuch as the outermost quills are also, as yet, non-existent; and there is a free finger-tip. But it is not nearly so long as in the hoatzin, and there is no terminal claw. Surely, from this, we may infer that the delayed development of the outer quills is a survival of a time when the ancestors of the pheasant were arboreal, and hatched their young in trees. Otherwise all the wing-quills should develop at the same time, and at the same rate. Here, then, is another instance of what can be learned of the past history of a bird by a careful scrutiny of the nestling. Sometimes we shall find our evidence in the wing, sometimes in some other organ. The sequence of plumage affords abundant evidence of this. But that is another story.

So much for the "intensive" study of the wing. A brief reference must now be made to the constantly repeated statement that nestling birds are "taught" to fly by their parents. There is no evidence whatever to support this belief: and much that goes to show its improbability.

Failing more suitable sites, sand-martins will often elect to build their nests in the crevices of the masonry of bridges.

From the mouth of this substitute for a burrow is often a sheer drop of many feet to the stream below. When the nestlings, fully fledged, leave their nursery for the first time they must either "fly" from the moment they take the first plunge from the masonry, or die. Failing to make the appropriate movements of the wings nothing can save them from a watery grave. There can be no "teaching" to fly. Indeed, death no less certainly awaits every house-martin when it plunges into space from the edge of the nest. The appropriate wing-movements, necessary to produce flight, in short, are "instinctive." Those with defective instincts are forthwith killed by falling to the ground. They leave no offspring to inherit their defects.

Perhaps the most convincing evidence of all as to the "instinctive" nature of flight, in nestling birds, is furnished by the mound-birds, of the Malay Region and Eastern Australia.

These extraordinary birds lay their eggs in heaps of decaying vegetable-matter, or in the soil near hot springs; and there leave them to their fate. They lay very large eggs, it is to be noticed, so large that the growing chick finds nourishment enough within the egg to enable it to pass the ordinary nestling stage while still within the shell. By the time it emerges it has both grown and shed its first coat of nestling-down, and has developed long wing-quills. Having burst its prison walls it wriggles its way up through the loose earth, to the light of day, ready to fight its way in the world unaided. Here, then, there can be no question of "teaching" the young to fly.

But some birds, at least, do, indeed, receive instruction when on the wing. And in such cases, it will be noticed, their food can only be captured by dexterous movements in full flight. For a day or two, for example, young swallows simply practice flight, to exercise and strengthen their wings. They are fed by their parents when at rest. The next step comes when they are fed on the wing, taking their food as they hover on trembling pinions from their parent's beak. In a little while the food is dropped as the parent passes, and the youngsters are made to catch it as it falls. From thence, onwards, they have to do their own hunting. The clumsy ones must die. Eagles and hawks, in like manner, teach their young to capture swiftly moving prey by dropping food to them in mid-air. If one fails to catch it the parent swoops down and seizes the hard-won meal before it reaches the ground; then mounting aloft with it, drops it once more, till, at last the required dexterity is gained.