For example, when the organic condition which leads to nest-building becomes active, the bird tentatively collects some of the necessary material in its bill, flies round with it, and then drops it. After a while it collects some more, and this time leaves it perhaps in a bush. Later on it makes another attempt, and, meeting with a situation which calls forth the appropriate response, it thereupon lays the foundation of the structure. We will assume that the nest is placed in the midst of a tangled bush. Well, the bird lays the first strands of the foundation and then goes in search of more material. The next 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.