12 THE RIDDLE OF MIGRATION 



bones. These do not add materially to the bird's 

 buoyancy, but they bring about an inrush of air 

 through the lungs and then out again, providing the 

 most perfect respiratory mechanism known in the 

 animal kingdom. Added to all this is the corre- 

 lated construction of the ribs, breast-bone and bones 

 of the pectoral girdle which together provide a con- 

 trivance that may be said to pump air in and out of 

 the lungs automatically with each wing-beat. 



From these comments, to which much might be 

 added, it will be apparent that the actual perform- 

 ance of continuous flight on the part of a bird is in 

 the nature of a pastime rather than a labor. From 

 the viewpoint of adaptive construction the typical 

 bird is undoubtedly perfectly equipped for life in the 

 air, more so than any of its fellow creatures. But 

 that, of course, is not all that is required. A 

 migrating bird, not only in the actual performance 

 of migration, but on the thousand and one occa- 

 sions of incidental events, exhibits reactions to its 

 environment that we know must depend on the pos- 

 session of a brain and nervous system, and before we 

 proceed further it is necessary that we take a 

 cursory survey of the possibilities and limitations of 

 the avian nervous system and related sense organs. 



The nervous system is made up of several com- 

 ponent parts. The dominating aggregation of 



