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.
A
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.