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.
[Illustration: _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.
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.
[Illustration: Gulls.]