6 THE RIDDLE OF MIGRATION 



expect, for no living creature offers so great a con- 

 trast to the cold, sluggish reptile of popular concep- 

 tion as the excessively active, hot-blooded bird. 

 Yet when we warm up a reptile by heating its sur- 

 roundings we frequently get an unexpected display 

 of activity. Many tropical reptiles, particularly 

 lizards are, as a matter of fact, among the earth's 

 speediest small animals. But a drop in temperature 

 distinctly crimps their style. Birds supply their 

 own heat (except in their extreme youth) and 

 remain constantly active. Their body tempera- 

 ture averages even higher than that of mammals 

 and their rate of metabolism is greater. It is the 

 source of their extraordinary activity and char- 

 acteristically restless nature and may, quite con- 

 ceivably, coupled with their power of flight and 

 potential disregard for barriers, be one of the chief 

 factors in inducing migrations. It is certainly one 

 of the important items that make migration a 

 possibility. 



Anyone who has kept birds in aviaries is ac- 

 quainted with their superabundance of energy. 

 Except when feeding, or during the breeding season, 

 they appear never to sit still but to fly incessantly 

 back and forth from one end of the aviary to the 

 other. Their daily mileage, although confined to 

 the limits of a cage, probably equals that of a normal 



