XVI 



LIFE IN THE AIR: THE BIRDS 



1 . Features of bird life 



The quality we define as 'life' is perhaps more fully represented in 

 birds than in other vertebrates, or indeed in any animals whatsoever. 

 It is difficult to find units by which accurate comparisons can be made 

 of such matters, but there is a meaning in the statement that the life of 

 a bird is more intense than that of, say, a reptile or a fish. Following 

 out our definition of the life of a species as the total of the activities by 

 which that particular type of organization is preserved, we shall find 

 that the birds have many and very varied activities, by means of which 

 a great deal of matter is collected into the bird type of organization. 

 Moreover, this is achieved under conditions remote from those in 

 which life first arose; the birds get a living by moving in the air, the 

 most difficult medium of all. 



Flight is of course the characteristic that gives us most fully the 

 feeling that the birds are active animals ; it impresses us as a technical 

 marvel and as a means by which the animals obtain a most enviable 

 and valuable freedom, enabling them to avoid their enemies and to 

 seek new habitats. Almost equally important items in the active life of 

 birds are the high and constant temperature and large brain. These 

 features have been acquired independently by birds and mammals, 

 and have led to profound changes in behaviour. A homoiothermic 

 animal does not need to change its activity with the changes in environ- 

 mental temperature; it can be continuously active, and, perhaps even 

 more important, its steady continuity of life makes possible the accurate 

 recording of past experience in the memory. Probably only with a high 

 and constant temperature can full use be made of the possibilities of 

 delicate balance of activities within large masses of nervous tissue. In 

 homoiothermic birds and mammals we find larger brains and more 

 elaborate social and family habits than in any other animals. 



2. Bird numbers and variety 



Flight necessitates a large surface-weight ratio, therefore birds do 

 not become so large as some mammals; nevertheless, an immense 

 biomass is produced by their very large numbers. Any attempt to 

 enumerate the bird population is largely guess-work, but the density 

 of breeding birds in different habitats in Britain has been estimated, 



