nges, whatever they may be, which are
responsible for the awakening of the sexual instinct.

In considering how this difficulty might be met, the importance of
migratory species as a channel of information was gradually borne in
upon me; for it seemed that the definiteness with which the initial
stage in the sexual process was marked off, as a result of the incidence
of migration, would go far towards removing much of the obscurity which
appeared to surround the earlier stages of the breeding problem in the
case of resident species. Recent observation has shown that I
exaggerated this difficulty, and that it is generally possible to
determine with reasonable accuracy the approximate date at which the
internal changes begin to exert an influence on the behaviour of
resident species also. Nevertheless, the specialised behaviour of the
migrants furnished a clue, and pointed out the direction which further
inquiry ought to take.

Those who are accustomed to notice the arrival of the migrants are aware
that the woods, thickets, and marshes do not suddenly become occupied by
large numbers of individuals, but that the process of "filling up" is a
gradual one. An individual appears here, another there; then after a
pause there is a further addition, and so on with increasing volume
until the tide reaches its maximum, then activity wanes, and the slowly
decreasing number of fresh arrivals passes unnoticed in the wealth of
new life that everywhere forces itself upon our attention. If now,
instead of surveying the migrants as a whole, our attention be directed
to one species only, this gradual arrival of single individuals in their
accustomed haunts will become even more apparent; and if the
investigation be pursued still further and these single individuals
observed more closely, it will be found that in nearly every case they
belong to the male sex. Males therefore arrive before females. This does
not mean, however, that the respective times of arrival of the males and
females belonging to any one species are definitely divided, for males
continue to arrive even after some of the females have reached their
destination; and thus a certain amount of overlapping occurs. A truer
definition of the order of migration would be as follows:--Some males
arrive before others, and some females arrive before others, but on the
average males arrive before females. This fact has long been known.
Gaetke refers to it in his _Birds of Heligoland