“After the cold season has passed, with the first fine days of spring the same birds return to the regions where they were born, making the journey this time in the opposite direction, from south to north. They take possession once more of their groves and forests, their rocks and prairies, which they know how to find with an inconceivable accuracy. There they build their nests, rear their young, and gain strength for the coming journey; and upon the return of cold weather they go back again to the lands of sunshine. “These periodical journeys are called migrations, of which there are two each year,—that of autumn, when the birds leave us and go southward, and that of spring, when they fly northward and come back to us. These semi-annual flittings take place all over the earth. “The various species do not all fix upon the same time for their migration, but each has its own calendar, from which it departs only very slightly. Some start well in advance of the increasing chill and the lessening abundance of food, while others do not leave their native land until driven by actual necessity, when the cold has become severe. Thus our martin flies away for Africa as early as the month of August, whereas the chimney-swallow lingers until October or even November. “The martins forsake our turrets and old walls, our steeples and belfries, while the summer heat is still intense and the small flies on which they feed are still abundant. It is not, then, any lowering of temperature that drives them away, nor is it any lack of food that hastens their departure; but they have a secret presentiment of the change of season that is coming in a few weeks; a deep-seated unrest, which they cannot overcome, warns them that the hour for their departure is drawing near. “If one desires to witness this anxiety that torments the bird when the time for migrating arrives, he may do so by rearing in captivity a migratory bird caught very young. The captive, though never having lived with its kind or had any knowledge of their migratory habits, and furthermore having been kept in a cage with no experience of cold or hunger, nevertheless, when the season for flitting arrives, shows agitation and mental distress, and tries to escape from its prison—after remaining so quiet and contented up to that time. Some inner voice—instinct we call it—says it is time to go, and the captive is eager to be off. If the desire is thwarted, death follows.