THE INFLUENCE OF DIFFERENT DAY LENGTHS 



ON THE TESTES OF A TRANSEQUATORIAL 



MIGRANT, THE BOBOLINK 



{DoUchonyx oryzivonis)"'- 



WILLIAM L. ENGELS 



Department of Zoology, University of North Carolina, Chapel Hill 



Birds that breed in one heniispliere and spend the nonbreeding season 

 in the other experience two periods of increasing day lengths every 

 year instead of one. Since they are exposed to two light cycles while 

 undergoing only one reproductive cycle, the question has often been 

 raised as to the possibility (or impossibility) of photoperiodism play- 

 ing a role in the cycle of their annual reproduction. Of the specula- 

 tions put forth, none as yet has been based on results of experiments 

 with transequatorial migrants, because, as far as I know, no such 

 experiments have been made. It is the complete lack of published 

 data in this important area that prompts me to report the following 

 experimental results, even though they are so obviously meager. 



THE LIGHT SCHEDULE IN NATURE 



The breeding grounds of the bobolink lie in eastern and central 



North America, mainly between 40" and 50° N. Lat.; breeding 



occurs chiefly in June and terminates in July. Before mid-August an 



exodus occurs, and the breeding grounds are essentially emptied 



before September. The vast majority of the population has passed 



beyond the southern shores of the United States by mid-October. Day 



lengths meanwhile have decreased from more than 15 hr (sunrise — 



sunset) to less than 12 hr (Fig. 1). The duration of the period of 



these short days depends on the time taken by the birds to reach the 



equator. I have been unable to find precise data on this point. 



* These studies were supported in part by a research grant (E-356) from 

 the National Institutes of Health, Pubhc Health Service, which was adminis- 

 tered by Charles E. Jenner. 



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