If man migrates he does so intelligently. Why not grant to birds the same faculty?
For the sake of illustration we will take the swallows, birds known to all, and describe their method of migration. Remember, that the old birds have been South, that they know the way and do not doubt their ability to pilot the young birds to the new home. They also know, from experience, the perils and hardships of a long flight while battling with wind and weather. Full well they know that young birds, just out of the nest, would not last a day's flight if raw and untrained. So they intelligently proceed to train the young birds into a suitable condition. Early in the morning, after the young are fed, they are marshalled along the wires and fences and drilled in the art of flying. At first they fly in small squads, just a family group, but later they gather into companies and practise until the companies are massed in one grand army corps. When the young birds are thoroughly drilled, that is, are hard of muscle and capable of keeping their place in the ranks, to touch elbows, as it were, the old birds are ready to lead the way South. To avoid straggling the departure is made in the night.
Up to this point we see no indications of instinct. The acts of the swallows are as intelligent as would be the acts of human beings under like circumstances. If a general had raw recruits to deal with he would drill them just as the swallows drill their raw recruits.
Perhaps the manœuvres of swallows gave mankind the idea of military tactics.
When we consider the journey of these birds South, why should we claim that their acts are guided by a supernatural power? Why not allow intelligence in flight as well as in preparing for flight?
We can readily understand how the old birds, that have made several journeys and must be familiar with all the landmarks, may make the journey without the aid of a supernatural power.
We must remember that the sense of sight in birds is developed to a degree unknown to mankind. It often happens that I startle a ruffed grouse from its perch in the night. In such case it hurls itself through the shrubbery with amazing speed. When I think of the keenness of sight that enables this bird to avoid twigs and limbs, I know that my sight is nothing but blindness in comparison.
Some birds fly high, and the earth is like a map beneath them, with a well-defined line between land and water. Birds that are familiar with the route ought to experience no difficulty in finding the way. Even the limited sight of man would serve unless handicapped by a dark, stormy night.
Young birds left to themselves will not go South. Young robins often get left in this vicinity. They are birds of the last brood usually; the parent birds are killed before the young learn to associate with the flocks in the neighborhood. They stay through the winter because they have no knowledge of the South and no guide to lead the way. Ducks hatched under hens from wild eggs will not go South. I once lived near a farmer that hatched out six black ducks. The farmer did not feed them, and they lived through the summer on a trout brook. In winter they huddled into a fence corner under some shrubbery. They had no instinct to send them South, although their flight feathers were perfect; but they possessed intelligence enough to seek the cattle tie-up for warmth whenever they found the door open.