ears, until by the beginning of
the eleventh year, we have 59,048 Yellow Buntings occupying 29,524-1/2
acres or 46 square miles. This, then, will be the result of the
operation of the impulse, providing that all the individuals survive and
that no complications supervene.

But of course complications are numerous, some of which retard while
others accelerate the rate of expansion. These complications arise from
various sources--in the first place from natural enemies which prey upon
the birds or upon their eggs; in the next place from climate which, if
it happens to be unfavourable, may mean that food is scarce and that
only a small percentage of the young survive; and lastly from
rivals--and by rivals I mean closely related forms that require a
similar station and similar food--which, by occupying available ground,
may check expansion, or, by forcing a continuation of the search, may
widen it.

Now when individuals fail as many do fail in their initial attempt to
secure territory, the activity of the impulse still persists, and there
is no control over the direction in which the bird continues to wander
whilst in search of its end. Some therefore seek in this direction,
others in that; some wander inwards into inhabited areas and fail to
find accommodation, or, according to the relative strength of their
impulse, perhaps succeed and so set free a new competitor, others wander
outwards into country uninhabited by the species. These latter we will
call "pioneers." They may find accommodation within a comparatively
short distance of their base, or they may come into competition with
rivals and fail, not necessarily on account of any congenital weakness
of ability, but because being warned by an alien song, they may be
precluded from coming into contact with just the environing conditions
which can supply the stimulus and allow behaviour to run its further
course--and so be obliged to extend their search into remoter districts.
But it must not be overlooked that they will be placed in a most
advantageous position so far as the attainment of reproduction is
concerned. In their search for territory they will meet with little
opposition and will be free to select whatsoever ground they will; and
be free also from intrusion by neighbouring males, which is so frequent
in occurrence and continues for so long in congested areas. Moreover, in
thinly populated districts, the pressure upon the available means of
support will not b