Each species presents a separate problem, to be solved for the most part only by patient, painstaking observation and by the recognition of subspecies."
The order in spring is yet unproved. "With many birds . . . the first individuals to appear in spring at a given locality are supposed to be old birds that nested there the previous year." These are followed by those which nested a little farther north, followed later by those Whose homes are in the most northerly part of the range. "If, then, for any species, the southern nesting birds lead the van in both fall and spring migration, and the near guard in each case is composed of northern breeding birds, it follows that some time between October and April at transposal of their relative positions occurs; and that the more southern birds pass over the more northern ones, which delay their migration, knowing that winter still holds sway in their summer dominions." It is not known where this transposal takes place, nor whether the northern birds remain in winter quarters till the southern birds have passed, or start a slow migration, during which the southern birds pass over them. Later another transposal occurs; the northern birds cross the southern part of the range, passing birds which are already nesting "Spring migration seems to be therefore for some species a game of leapfrog—the
