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.
Gulls.