EFFECT OF EXPOSURE ON THE YOUNG 179 



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 fii'st 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 — 



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