Birds in Flight

by W. P. Pycraft

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.

Grouse

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.