BEITISH WARBLERS 



under my notice where there appeared to be little doubt that 

 the young did not all desert the nest at the same moment. 

 The explanation must be that the stronger individuals, receiv- 

 ing an unequal proportion of the food, develop more rapidly, 

 and thus are capable of leaving the nest earlier. But what do 

 we mean by the stronger individuals ? Is it an instance of the 

 natural selection of those that are more fit ? It is a common 

 saying amongst those who are accustomed to take the young 

 of such birds as the Magpie (Pica rustica), while still in the 

 nest, in order to make pets of them, that the females are at 

 the bottom of the nest, and the males on top. How far this is 

 correct I have had no means of judging, but it may be a possible 

 explanation. The males would be naturally the stronger, and 

 would therefore require and receive a greater supply of food, 

 and, being the stronger, would struggle until they forced them- 

 selves above the females in their efforts to reach the food as it 

 was from time to time brought by the parents. The explana- 

 tion of the behaviour of the parents and young may therefore 

 be this : that the stronger individuals — possibly the males — 

 require more food and thus are more persistent in stretching 

 up their necks and opening their gapes. And I believe that 

 no one could watch a nest of young from day to day without 

 coming to the conclusion that the most persistent individuals 

 w T ere the ones that on the average secured a larger quantity of 

 food, and that this persistency was the direct factor which 

 influenced the behaviour of the parents ; the indirect deter- 

 mining factor being the relative rapidity of digestion in the 

 different individuals and the consequent sensation of hunger. 

 To take an extreme case : a bird that has received a large 

 supply of food will lie quietly in the nest with a tendency to 

 sleep, and when the parent again arrives with food and the 

 remainder of the young stretch out their necks and utter their 

 call-note, will pay no heed, but will remain in an apparently 

 sleepy condition. As the process of supplying food continues, 

 it will gradually rouse itself, at first stretching out its neck 

 with little determination, but finally becoming frantic in its 



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