54 MANNER OF MIGRATION. 4. Transient Visitants. — Birds whose summer home is north and whose winter home is south of us. In traveling from one to the other they pass through the intervening region as " transients." Manner of Migration. — The Oriole, who builds his swinging nest in your elm tree, will winter in Central America ; the Bobolink, who seems so care-free in your meadows, must journey to his winter quarters in southern Brazil. But, unless accident befalls, both birds will re- turn to you the following spring. We are so accustomed to these phenomena that we accept them as part of the changing seasons without realizing how wonderful they are. But look for a moment at a map, and try to form a mental picture of the Bobolink's route. Over valleys, mountains, marshes, plains, and forests, over straits and seas hundreds of miles in width, he pursues a course through trackless space with a regularity and certainty which brings him to the same place at nearly the same time year after year. How much of his knowledge of the route he has inherited, and how much learned dur- ing his own lifetime, is a question we may return to later ; now we are concerned with actual methods of migration. Immediately after, or even during the nesting season, many birds begin to resort nightly to roosts frequented sometimes by immense numbers of their kinds, with often the addition of other species. These movements are apparently inaugurated by the old birds, and are in a sense the beginnings of the real migratory journey. Other birds roam the woods in loose bands or families, their wanderings being largely controlled by the supply of food. During this time they may be molting, but when their new plumage is acquired they are ready for the start. The old birds lead the way, either alone or asso-
