216 MIGRATION 



significance of the altitudinal migration in the Rocky Mountains and 

 the Sierra Nevada. Here there can be no possibility of the influence 

 of day length. 



From the synthetic and objective approach to the problem of 

 migration it appears manifest that metabolic condition is the crux of 

 the matter, especially as influenced by hormones, vitamins, blood 

 physiology, temperature, and food consumption in terms of length 

 of daylight and darkness. This might well be considered to be axio- 

 matic were it not for the diversity of hypotheses and the fact that 

 experiment has as yet been too little focused upon it with respect to 

 migration. The chief contribution is that of Kendeigh, on the basis 

 of which he is justified in stating that the critical prerequisite to 

 southward migration is the average night temperature combined with 

 the number of hours of darkness for which the bird is without food. 

 It is obvious, however, that only the first can apply to movement from 

 higher to lower altitudes. 



As to the northward migration, the chief factor appears to be 

 high air temperature in the form of the daily maximum. The food 

 supply plays an indirect part inasmuch as wider areas and longer 

 days will be requisite for the acquisition of territories and the feeding 

 of the young. By reason of the high body temperatures of birds, the 

 margin of safety in the upward direction is much narrower, and this is 

 likewise true of the range within which the food-getting activity is 

 comfortable or possible. It is a matter of common knowledge that 

 activity is greatly reduced during afternoon maxima or hot periods 

 in summer, and Kendeigh has brought together some of the outstand- 

 ing examples of this response (1934:344). Perhaps the most striking 

 illustration is afforded by such ground birds as the horned lark and 

 meadowlark, which follow the moving shadow of the fenceposts on 

 the Great Plains where other shade is lacking. The evidence drawn 

 from repeated observations of this response is supported by Riddle 

 et al, {loc. cit.) who find that the metabolism of doves is so reduced 

 at an air temperature of 86° as to lead to abnormal functioning, as 

 well as by Kendeigh (1934), who determined a temperature of 93° F. 

 to be critical, curtailing activity and interfering with normal behavior 

 and reproduction. 



The relatively enormous amount of food needed to maintain a bird 

 in normal activity lies at the bottom of the metabolic condition for 

 migration, as it is also for tolerance of winter conditions by perma- 

 nent residents and throughout the reproductive cycle for all species. 

 One function assigned to the establishment of territories during the 



