its
presence by song. The male resident passes only the earlier part of the
day in its territory at the commencement of the period of occupation;
the male migrant remains there continuously from the moment it arrives.
The male resident deserts its territory at intervals, even in the
morning; the male migrant betrays no inclination to do so. Thus there is
a very close correspondence between the behaviour of the two, and what
difference there is--slight after all--cannot be said to affect the main
biological end of securing territory. One is apt to think of the problem
of migration in terms of the species instead of in terms of the
individual. One pictures a vast army of birds travelling each spring
over many miles of sea and land, and finally establishing themselves in
different quarters of the globe; and so it comes about, I suppose, that
a country or some well-defined but extensive area is regarded as the
destination, the ultimate goal, of the wanderers. But the resident male
has a journey to perform, short though it may be; it, too, has a
destination to reach, neither a country nor a locality, but a place
wherein the rearing of offspring can be safely accomplished, and it,
too, arrives in that place in advance of the female.

With these facts at our disposal, we will endeavour to find an
explanation. It is unlikely that specialised behaviour would occur in
generation after generation under such widely divergent conditions,
and, moreover, expose the birds to risk of special dangers, if it were
but an hereditary peculiarity to which no meaning could be attached.
Hence the appearance of the males in their breeding haunts ahead of the
females becomes a fact of some importance, and suggests that the
extensive journey in the one case, and the short journey in the other,
may both have a similar biological end to serve.

Darwin evidently attached importance to this difference between the
males and the females in their times of arrival. In the _Descent of Man_
he referred to it as follows: "Those males which annually first migrated
in any country, or which in spring were first ready to breed, or were
the most eager, would leave the largest number of offspring; and these
would tend to inherit similar instincts and constitutions. It must be
borne in mind that it would have been impossible to change very
materially the time of sexual maturity in the females without at the
same time interfering with the period of the production