Page:The British Warblers A History with Problems of Their Lives - 5 of 9.djvu/49

 be eliminated just as surely as one whose members did not secure sufficient ground to ensure an ample supply of food for their young. But it is clearly impossible to trace the history of the development of the breeding territory unless we have before us a true phylogeny, and the conditions under which the earlier types existed. Many of the sea-birds, which breed upon ledges of rock, are compelled to travel some distance before they can find a situation suitable for rearing their offspring, whereas the question of food presents no great difficulty; consequently the limits of their territory have been reduced to the minimum size necessary for reproduction. In a case in which the territory has been developed in order to ensure an ample food supply during the period of reproduction, it is possible that the area of that territory, and consequently the numerical strength of the species, will be found to be proportionate to the distance its members are accustomed to travel in search of food. This, in some instances at least, appears to be the case. Those species that have long distances to travel are represented by comparatively few, and conversely those that find food close at hand by numerous members. The birds of prey wander over a great distance, consequently their territories are large, and the same may be said of the Raven. It is not every tree that can supply the necessary food for the Woodpecker—in fact, such trees are relatively scarce—therefore it has first to search for and find the suitable trees, and then it will visit them daily, generally in routine, covering a considerable area while so doing. At the same time it by no means exhausts all the possible food supply; the area covered would possibly be sufficient to maintain one other pair at least, but by reason of the fact that the food is difficult to find the distance traversed may be beyond what is really necessary. In such a case the conditions of existence might be so strenuous that it would become of the utmost importance to any particular individual to prevent, not only members of its own species but also those of closely allied species, from breeding in the