MIGRATION OF BIRDS 



209 



and others to the effect that all aspects of the problem are susceptible 

 of solution by research, in which experiment must take the leading 

 role. 



The range of migration in birds varies from the shifting of much 

 of the population of those species called permanent residents over most 

 of their range, southward in autumn and northward in spring, to the 

 migration of all the population of certain species such as the golden 

 plover from the Arctic region to Patagonia and back each year. The 

 bluejay, regarded as a permanent resident from Newfoundland to 



Isotherm of 35" F. 



Isochronal Migration Lines 



Fig. 50. — Migration of the Canada goose. The northward movement keeps pace 



with the advance of spring, in this case the advance of the isotherm of 35° F. 



agreeing with that of the birds. (After Lincoln, 1935.) 



Florida, is an example of shifting. Birds banded in Illinois and Iowa 

 during the summer have been caught 200-300 miles south of the band- 

 ing station during the cooler months. Groups of individuals are also 

 often seen working south through the trees in the cool days of autumn 

 (Lincoln, 1933, 1935). See Figs. 49-51. 



The study of migration is at present passing from the first period, 

 that of observation and speculation, into one of experimentation. Such 

 a preliminary phase was as indispensable as it was inevitable, and 

 the student of migration, when beset by the many conflicting views, 

 should not overlook the great services rendered by the observers of 

 this epoch. They have not merely builded a broad foundation of 

 incontestible facts, but they have in addition brought forward all pos- 

 sible explanations and insured their consideration. Moreover, ornithol- 



