tributory factors, the failure of any one of which would mean failure of the whole. For a bird, success in the attainment of reproduction does not merely imply the successful discharge of the sexual function; much more is demanded; it must find somewhere to build its nest and to lay its eggs, it must shield its young from extremes of temperature and protect them from enemies, and it must be in a position to supply them with food at regular intervals. And, consequently, every situation is not equally favourable for rearing young; there must be a plentiful supply of food of the right kind in the immediate vicinity of the nest, and it must be in greatest abundance just at the moment when it is most urgently needed—that is to say, during the first few weeks after the birth of the young. Success, therefore, depends upon manifold relationships which centre in the station, and these relationships vary in intensity with the conditions of existence.
First, then, let us examine the problem from the point of view of the food-supply. There are many species whose success in rearing offspring is largely dependent upon the rapidity with which they can obtain food; and it makes but little difference which species we choose out of many—Finch, Bunting, Warbler, or Chat. I shall choose the Buntings, as their life-history in broad outline conforms to the general type, and, moreover, their behaviour is fresh in my mind. The young are born in a very helpless state; they are without covering—-
