680 REPRODUCTION AND MIGRATION IN BIRDS 



eluded that the southward migration in fall appeared to be inde- 

 pendent of the influence of the gonads. 



Rowan's epochal work defined the basic problems and stimulated 

 an experimental attack on the regulation of bird migration and 

 gonadal cycles. Dr. Farmer (p. 717) has reviewed the progress which 

 has been made on the photoperiodic control of annual gonadal cycles. 

 He has also reviewed the physiological aspects of the stimulus for 

 migration (1955). My studies and interest have centered around the 

 problem of the stimulus for spring migration, and in this report I 

 shall consider primarily the role of the external factor, day length. 

 Since migratory behavior in the spring is closely linked with the re- 

 crudescence of the gonads, the role of light and darkness in the regu- 

 lation of the annual gonadal cycle will also be considered. Complete 

 reports of the unpublished studies from our laboratory which are cited 

 in this paper will be published later elsewhere. 



The first extensive series of experiments which were designed to 

 test Rowan's hypothesis were performed by Wolfson (1940, 1942, 

 1945) in Berkeley, California, using the Oregon junco, a species 

 closely related to the slate-colored junco. These experiments cor- 

 roborated Rowan's observation that birds could be stimulated to 

 migrate northward months ahead of time by subjecting them to 

 artificial increases in day length in the late fall and winter. However, 

 they also demonstrated that birds in breeding condition would mi- 

 grate, which was contrary to Rowan's conclusion. Wolfson found 

 that in addition to gonadal growth preceding migration, there was 

 also a marked increase in body weight caused by large deposits of 

 subcutaneous and intraperitoneal fat, which was a better criterion of a 

 readiness to migrate than the condition of the gonad. Later studies 

 demonstrated that the pituitary was also involved in this premigratory 

 change in physiological state. Members of a nonmigratory race of 

 the same species, which were exposed to the same environmental con- 

 ditions in nature and in the laboratory, differed from migratory indi- 

 viduals in not showing marked deposition of fat or increase in body 

 weight and in having a much faster rate of gonadal development which 

 resulted in an earlier breeding season. 



The relation between physiological changes induced by increasing 

 day length and the actual release of migratory behavior are not known, 



