304 THE BIOLOGY OF BIRDS 



and of some spines in the mouth. The feeding of the young 

 obviously involves a great expenditure of energy, for it is 

 often necessary to fly several miles before the fishing-ground 

 is reached. This is simply one instance out of hundreds, 

 which lead us to agree with Cresson (19 13) that there has been 

 inadequate appreciation of the amount of time and energy 

 that parent animals, and birds very notably, expend on 

 securing the welfare of the young. We have read that on a 

 long summer day a pair of Blue Tits spent sixteen hours in 

 bringing over a thousand caterpillars and grubs to their 

 hungry nestlings. 



There are some interesting adaptations on the part of 

 nestlings that make feeding easier. In many cases, especially 

 when the parent birds have to go to and fro hundreds of 

 times in a day with tiny mouthfuls of insects, it is very 

 important that the moment of feeding should be as brief as 

 possible. So the nestling opens its mouth, not voluntarily, 

 but reflexly (as we draw back our finger from something very 

 hot) when its bill is touched with the food its parent brings. 

 It is highly probable, as Mr. W. P. Pycraft suggests, that the 

 occasionally bright colour of the nestling's mouth may 

 facilitate the precision of the parent's touch, especially in 

 dim light. Fumbling is out of the question, and we can 

 understand from this simple case how an apparently trivial 

 variation (in this case a brightly coloured mouth) might have 

 survival-value and be established in the course of the struggle 

 for existence. 



There is no end to the subtlety of parental-care adapta- 

 tions. How fit it is that young woodpeckers, hatched out in 

 a deep hole in a tree, should have their juvenile claws and 

 muscles particularly well suited for clambering up to the 

 entrance, thus to receive with the least possible loss of time 

 what their parents bring. 



Besides keeping the nestlings hidden, warm, and well 

 fed, the parent birds have a more commonplace task which 

 only a few neglect — keeping the nest clean. They often 

 remove the voided matter in their bills, an operation made 

 easier by a thin pellicle of mucus, which forms a sort of 



