Page:The British Warblers A History with Problems of Their Lives - 5 of 9.djvu/68

 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, receiving 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 themselves above the females in their efforts to reach the food as it was from time to time brought by the parents. The explanation 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 were 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 determining 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