Page:The British Warblers A History with Problems of Their Lives - 5 of 9.djvu/47

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are different, aquatic insects being then plentiful, and young succulent aquatic plants abundant. Or take the case of the migrants after the young are reared and able to take care of themselves: the parent birds do not still remain in their territory, but frequently separate and wander from place to place with no fixed abode. It cannot therefore be the desire to secure an efficient supply of food for themselves that causes the Moorhens to defend their territory so stoutly, since they seek for and find much of it upon neutral ground. Nor can it be anxiety to prevent the possibility of lack of food for themselves that impels the Warblers to struggle together, for why then should they desert the territory they have won and so consistently defended as soon as the young reach a certain age? As I shall presently mention, the Reed Warblers do seem to regard the tops of the alders surrounding the reed bed as neutral ground, resorting thereto to seek food for their offspring. But their territories are small owing to the comparative scarcity of suitable breeding grounds, and some relaxation of the general law may be advantageous in their case.

Glancing at an early period in the history of bird life, we are now in a position to understand partly what may have taken place. Those members that were not pugnacious, and thus allowed others to breed in proximity to them, would certainly attain to reproduction, but their offspring, if they did survive, would be weak owing to lack of food and exposure to changes of temperature, and in their turn would produce a weakly race unfitted to struggle, when called upon to do so, with stronger individuals, and so the tendency to sociability, having in such cases no opportunity for expansion, would gradually disappear. But the necessity for a supply of food in the immediate vicinity of the nest would not always have been imperative, and in some cases might not have been of sufficient importance to outweigh advantages gained from a number of individuals congregating together, so that we should then find members of one species breeding in a community.