THE ROLE OF LIGHT AND DARKNESS IN THE 



REGULATION OF SPRING MIGRATION AND 



REPRODUCTIVE CYCLES IN BIRDS ^ 



ALBERT WOLFSON 

 Northwestern University, Evanston, Illinois 



DAY LENGTH AND THE STIMULUS FOR SPRING MIGRATION 



Experimental and physiological studies of the regulation of reproduc- 

 tive and migratory cycles in birds were initiated by Rowan (1925, 

 1929), who discovered that gonadal recrudescence could be induced 

 out of season, in late fall and winter, by subjecting slate-colored 

 juncos iJunco hyemalis) to artificial increases in day length. On the 

 assumption that migratory behavior in the spring, when birds are fly- 

 ing north to their breeding grounds, was a phase of sexual behavior. 

 Rowan tested the effect of experimentally induced gonadal recrudes- 

 cence on migratory behavior by releasing juncos in winter, many 

 months ahead of the normal time of their spring migration and with 

 their gonads at various stages of development. From the results of 

 these experiments, Rowan concluded that in the slate-colored junco, 

 and other fringillids, the stimulus to migrate in the spring was regu- 

 lated by external and internal factors. The external factor was the 

 increasing day length after December 2 1 . The internal factor was the 

 production of sex hormones, which he correlated with the recrudesc- 

 ing gonads of spring and the regressing gonads of fall. The normal 

 winter minimal gonad and the normal maximal gonad of the breeding 

 period showed httle or no hormone-producing interstitial tissue, and, 

 hence, would not stimulate migratory behavior. From the results of 

 later work with crows {Corvus brachyrhynchos) Rowan (1932) con- 



1 The investigations reported in this paper have been supported by the 

 Graduate School of Northwestern University, the Society of Sigma Xi, and the 

 National Science Foundation. I acknowledge with sincere gratitude the able 

 assistance and valuable contributions of Hudson S. Winn, Max Shank, Tom 

 Kemper, Ormsby Annan. Betty Annan. Bruce Belshaw. and Larry Greenburg 

 to the research program in our laboratory. 



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