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 feet of ledge correspond to the tree or clump of bushes which acts as a headquarters for the Warbler, and the occupation of them is determined, as it is in the case of the Warbler, by racial preparation. Since, however, the ledge is only made use of for the immediate purpose of incubation and is in no way affected by questions relating to food, there is no occasion for the bird to wander along the ledge nor to encroach upon those adjoining. Hunger stimulates the Warbler to search the surrounding trees, and so to extend its area; but hunger takes the Guillemot down to the water, and hence the area which it primarily occupied remains unmodified.
