8 THE BIOLOGY OF BIRDS 



particularly important that the movements should be quick 

 and precise. Furthermore, when birds became fliers, able 

 suddenly to evade their enemies by rising into the air, they 

 regained, as it were, the many-sided possibilities of loco- 

 motion that free-swimmers enjoy. And the brain is in 

 great part an organ for the co-ordination of movements. 



Perhaps, however, one may go deeper. There is no 

 doubt that birds express one of the trends of organic evolution 

 which may be described as the economising of reproduction. 

 They have usually a sharply punctuated reproductive 

 period and a small family. The flying habit has made it 

 possible to deposit the eggs in safe places, and variations in 

 the direction of reduced reproductivity must have been 

 correlated from the first with variations in the direction of 

 increased parental care. These are indissoluble except in 

 very peculiar cases. The comparative safety secured for 

 the eggs and young allowed of a long period of development 

 before hatching, a possibly long sheltered period of infancy 

 in the nest, and a possibly long nurture period in which 

 the capacities of the young creatures are educated and new 

 departures tested. All this surely favours the improvement 

 of the brain and its correlated mentality. 



With the alertness that is as characteristic of most 

 birds as it is essential may be associated the fine development 

 of the senses of sight and hearing. 



Always returning to the heart of the matter, which is 

 the power of flight by means of feathers, we can understand 

 why flying creatures should have a sharply punctuated 

 reproductive period. It is plainly advantageous for the 

 female at least (and the males would have to follow) not to 

 have a large ovary throughout the greater part of the year. 

 As a matter of fact, the gonads sometimes dwindle almost 

 out of sight. Now this punctuation of the reproductive 

 period probably means greater intensity of the associated 

 emotions between mates, and may be correlated with that 

 heightening of feeling which overflows in song. Given 

 considerable vocal powers, originating as sex-calls and 

 elaborated into courtship ritual, there is an opening up of 



