CONCLUSION 217 



Conclusion 



In concluding these observations, we may define 

 migration among birds briefly as advance and retreat 

 with fluctuation in conditions favorable to the vari- 

 ous species. Each form has its own reaction to its 

 environment, so that the movement found in migrant 

 forms differs widely, and seldom do two present the 

 same picture. Dr. J. Grinnell ^ has defined migra- 

 tion as "a phase of distribution wherein more or 

 less regular seasonal shifting of population takes 

 place in response to precisely the same factors as 

 hem in the ranges of sedentary species.'' It appears 

 that the beginnings of the present instinct for mi- 

 gration, and the habit of its continuance, are so 

 ancient that they are wholly obscure and may be 

 interpreted only in terms of present conditions. The 

 underlying cause is certainly complex and is due to 

 multiple factors. We have in the past fifty years 

 cleared away many uncertainties regarding it, but 

 must look to the future to explain definitely the 

 basic reasons for the institution of migration (par- 

 ticularly in species that seem to have no need for 

 seasonal movement), the cause of the varying 

 length of the journey in different forms, and the 

 method of orientation followed in pursuing flight 

 over courses which, to young individuals at least, 

 are unknown. 



' Auk, 1922, pp. 379-380. 



