PHOTOPERIODISM AND THE ANNUAL TESTICULAR CYCLE OF THE BOBOLINK (DOLICHONYX ORYZIVORUS), A TRANS-EQUATORIAL MIGRANT, AS COMPARED WITH TWO TEMPERATE ZONE MIGRANTS

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Abstract
1. Bobolinks exposed to 10-hour photoperiods daily through the fall, winter and spring did not develop the beak pigmentation characteristic of the male nuptial dress until early in May. Others exposed at the same time to 14-hour photoperiods daily failed to develop beak pigmentation. But when exposure to 14-hour photoperiods was preceded by 8 weeks (October-November) of 10-, 11- or 12-hour photoperiods daily, pigmentation of the beak began variously between mid-February and mid-March. 2. Juncos and white-throated sparrows exposed to 14-hour daily photoperiods following 8 weeks (October-November) of 10-hour daily photoperiods had completed spermatogenesis by mid-January; but in others for which the preliminary photoperiod had been 12 hours the testes remained inactive. 3. Bobolinks held throughout the fall, winter and spring in an outdoor aviary, on changing day-lengths natural at Lat. 36° N., developed beak pigmentation in April, thus being approximately in phase, in their testicular cycles, with locally wintering juncos and white-throated sparrows. 4. The results are interpreted to mean that bobolinks, unlike typical Temperate Zone migrants, are able to overcome post-nuptial photorefractoriness on days of intermediate length, such as would be encountered during transequatorial migration, but that regrowth of the testes through the long days of the southern hemisphere summer proceeds at a rate much slower than that demonstrated by Temperate Zone migrants on similarly long days, and that full recrudescence is thus delayed, probably beyond the onset of northward migration. 5. It is further suggested that as far as the photoperiodic mechanism is concerned bobolinks could establish a wintering population in the northern hemisphere; but juncos and white-throated sparrows could not extend their wintering range into the southern hemisphere without readjustment of this mechanism.

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