158 THE BIOLOGY OF THE SEASONS 



ture of the surrounding world, and when they cool below 

 a point, which differs for different species, they cease to 

 develop and soon die. The nest often economises the animal 

 heat of the brooding bird, so that there is little waste. 

 Thus the accomplishment of the development within a 

 suitable time is secured. Mr. Pycraft starts the evolution 

 of nests with birds who gathered a small heap of grass and 

 sticks, and thus kept themselves and the eggs warm and dry 

 when the ground was damp. It is also evident that the 

 nest is convenient for feeding purposes for the prolonged 

 gastric education on readily digestible food which many 

 young birds require. It is often a convenient temporary 

 prison for the nestlings, so that they are less liable to 

 imperil their lives by making premature excursions. 

 Sometimes the nest serves for a certain amount of home- 

 education. Furthermore, the nest is often a comfortable 

 and secure resting-place for the brooding bird. The breeding 

 period is a climax of activity ; it is soon to be followed by the 

 effort of migration ; the need for rest is obvious. 



Dr. Alfred Russel Wallace has elaborated the theory, for 

 which there is much very interesting evidence, that when 

 both the cock- and hen-birds are of strikingly gay and 

 conspicuous colours, as in many kingfishers, the nest is such 

 that eggs and young and sitting birds are concealed from 

 view ; while, whenever the male is gay and conspicuous 

 and the nest is open so as to expose the sitting-bird to view, 

 the female is dull or obscure in her coloration. He shows 

 how this relation might come about in the course of Natural 

 Selection. 



We usually associate a nest with - concealment or in- 

 accessibility, with some degree of privacy. But there are 

 familiar exceptions. A strong race, like that of the rooks, 

 can afford to indulge their social tendencies even in nesting, 

 and their conspicuous nests challenge attention, We find 



