Granting this, however, as the first cause, we are only on the threshold; the question still remains unanswered, what actually impels the birds to seek fresh food supplies or to look for safe nesting places? The natural answer, the cravings of nature and sexual impulses fails to give satisfaction in every case. Wanderings in search of food might lead in any direction, and probably did in the first place, but new birds in the main travel south in search of food and north in search of home, and many of them perform immense journeys, passing over or through lands which are capable of supporting a wealth of bird-life even in the winter months.
The majority of Arctic birds or those nesting in high latitudes leave before the great harvest of autumn fruits, and even our common swift begins to depart—for all do not go at once—towards the end of July, when insects are more abundant than at any other time of the year. Food supply has not failed when most birds start their journey in search of food! Again in spring, when it is claimed that the powerful sexual impulses are sufficient reason to account for the northward journey, hosts of sexually immature birds and of others which are apparently mature but do not breed that spring, migrate northwards, some even arriving before the mature birds of their own species.
