encroached upon at
the risk of a conflict--all of this can be observed with little
difficulty. But if we regard them as so many lines definitely delimiting
an area of which the bird is cognisant, we place the whole behaviour on
a different level of mental development, and incidentally alter the
complexion of the whole process. It would be a mistake, I think, to do
this. Though conscious intention as a factor may enter the situation,
there is no necessity for it to do so; there is no necessity, that is to
say, for the bird to form a mental image of the area to be occupied and
shape its course accordingly. The same result can be obtained without
our having recourse to so complex a principle of explanation, and that
by the law of habit formation. In common with other animals, birds are
subject to this law in a marked degree. An acquired mode of activity
becomes by repetition ingrained in the life of the individual, so that
an action performed to-day is liable to be repeated to-morrow so long as
it does not prejudice the existence or annul the fertility of the
individual.

Let us see how this may have operated in determining the limits of the
area acquired, and for this purpose let us suppose that we are observing
a male Reed-Bunting recently established in some secluded piece of marsh
land. Scattered about this particular marsh are a number of small
willows and young alder trees, each one of which is capable of providing
plenty of branches suitable for the bird to perch upon, and all are in a
like favourable position so far as the outlook therefrom is concerned.
Well, we should expect to find that each respective tree would be made
use of according to the position in which the bird happened to find
itself. But what actually do we find--one tree singled out and resorted
to with ever-increasing certainty until it becomes an important point in
relation to the occupied area, a headquarters from which the bird
advertises its presence by song, keeps watch upon the movements of its
neighbours, and sets out for the purpose of securing food. We then take
note of its wanderings in the immediate vicinity of the headquarters,
especially as regards the direction, frequency, and extent of the
journeys; and we discover not only that these journeys proceed from and
terminate in the special tree, but that there is a sameness about the
actual path that is followed. The bird takes a short flight, searches a
bush here and some rushes there, r