TESTES OF TRANSEQUATORIAL MIGRANTS 765 



photoperiodic mechanism of bobolinks is indeed adapted to this re- 

 quirement. 



All in all, the results of these experiments are quite compatible with 

 the hypothesis advanced by Farner (1954) concerning photoperiod- 

 ism and transequatorial migrants. The essential points in this hypothe- 

 sis include (1) "a characteristically longer refractory period"; (2) the 

 escape from short autumn days and the retarding effect on refractory- 

 period release of the lengthening days of the Southern Hemisphere 

 spring; and (3) the eventual release from photorefractoriness, some- 

 time after the southern solstice, and subsequent photostimulation on 

 the still long, even though shortening, days of the Southern Hemi- 

 sphere summer. 



SUMMARY 



Experiments with the bobolink, a transequatorial migrant, demon- 

 strate the existence of a postbreeding photorefractoriness which is 

 maintained by continuously long days but which is broken by a period 

 of short days. Birds exposed to 14-hr photoperiods after 8 weeks on 

 10-hr photoperiods show testicular response several weeks earlier than 

 do birds on the same 14-hr day after 8 weeks on 12-hr photoperiods. 

 This is interpreted to mean that full recovery from photorefractori- 

 ness occurred on the long day, the time of its occurrence depending on 

 the nature of the previous exposure to short days. The data and their 

 interpretation are in accord with the day length cycles experienced by 

 bobolinks in nature. 



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Burger, J. W. 1949. A review of experimental investigations on seasonal repro- 

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Engels, W. L., and C. E. Jenner. 1956. The effect of temperature on testicular 

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Farner, D. S. 1954. Northward transequatorial migration of birds. Sci. Rev. 

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Miller, A. H. 1955. The expression of innate reproductive rhythm under con- 

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Pfeiffer, C. A., C. W. Hooker, and A. Kirschbaum. 1944. Deposition of pig- 



