CHAPTER VIII. 



CONDITIONS OF FLIGHT. 



Routes of Migration, how followed by Birds — Paley's Definition 

 of Instinct — Impulse of Migration — Restlessness of Captive 

 Birds — Certain Routes followed by Certain Individuals — 

 How a Route of Migration has been Learnt — Mysterious 

 " Sense of Direction " a Myth — Altitude of Migration' Flight — 

 Advantages of a Lofty Course — The Order of Migration — A 

 few Old Birds Migrate as Early as the Young — The Daily 

 Time of Migration — Amount of Sociability amongst Birds on 

 Passage — The Perils of Migration. 



In the preceding chapter we endeavoured to ascertain 

 how a Route of Migration has been formed ; it now 

 becomes necessary to inquire how birds continue to 

 follow those routes so unerringly, how they manage to 

 traverse them for such long distances apparently with 

 so few mistakes. If we define Instinct as Paley did, 

 and describe it as " a propensity prior to experience and 

 independent of instruction," which I think is about as 

 good a definition of the power as we can ever hope to 

 possess, then most assuredly Instinct can never control 

 the performance of avian season-flight. The impulse of 

 migration may be, and probably is, a very deeply rooted 

 one, an hereditary impulse even. Captiv^e birds in which 

 the habit of migration is dominant, have often been 

 observed to become exceedinglv restless as the usual 



