t time it approaches
the nest from the opposite side of the bush, and presently it finds yet
a third entrance. But each entrance is not made use of in turn: one is
employed more frequently than the other two, and in the course of time
becomes the sole highway to and from the nest. Suppose now that, when
the young are hatched, I cut away the foliage from the bush on the
opposite side from that on which the bird customarily enters, and by so
doing leave the nest exposed, what is the result? The female arrives
with food, threads her way through the bush, and, when beside the nest,
pauses as if aware that some change had taken place, and then flies away
through the new opening. In a short time she returns, flits from twig to
twig on the outskirts of the bush, and comes upon the new opening--there
she hesitates. But though the nest is in full view and within a few
inches of her perch, and though the young stretch out their necks, yet
so strong is the former habit that she is compelled to return to the
opposite side and approach the nest by the usual circuitous route before
she distributes the food amongst her offspring.

Let us see how far this law of habit formation may have been effective
in defining the extent of the area occupied. When a male Warbler arrives
at its destination in the spring it seeks out a suitable environment,
and, having found a place unoccupied by any other male, settles in it
and remains there--its behaviour up to this point being determined by
racial preparation. After the fatigue of the journey its movements are
at first sluggish; hunger, however, asserts itself and a search is made
for food; wandering away from the position in which it first settled and
which acts as a headquarters, it hunts through certain trees here or
certain bushes there and returns, and presently it wanders away again,
perhaps in another direction, but, as before, works its way back again
to the headquarters. The journeys thus radiate outwards from the
headquarters, and according to the success with which the bird meets,
so, probably, it happens that some trees are searched more often than
others and certain directions are taken more frequently than others, and
by constant repetition a routine is established which limits the
direction and scope of its wanderings.

But in the case of the Guillemot the conditions of existence are
reversed: food can be had in abundance but suitable breeding stations
are scarce. The few square fe