DAY-LENGTH AND TERMINATION OF PHOTOREFRACTORINESS IN THE ANNUAL TESTICULAR CYCLE OF THE TRANSEQUATORIAL MIGRANT DOLICHONYX (THE BOBOLINK)

¹ Department of Zoology, University of North Carolina, Chapel Hill, N. C.

1. The testicular cycle of Dolichonyx orysivorus, a bird which breeds above Lat. 40° N. and winters below Lat. 8° S., exhibits a photorefractoriness in early autumn, which is maintained by constant daily 14-hour photoperiods (experiment begun October 2).

2. Some individuals which had been held captive outdoors, exposed to the natural day-length of Lat. 36° N., were shifted to experimental, long photoperiods at various times between November 1 and November 28. Within a few months all of them developed the characteristic black beak pigmentation indicative of testicular recrudescence. From these results it is concluded that, under the influence of autumnal day-lengths of middle latitudes of the northern hemisphere, refractoriness is terminated by November 1 and that Dolichonyx thus does not differ appreciably from temperate zone species in the timing of this event under these conditions.

3. From published data on the time of autumnal migration within the United States and from data furnished by museum specimens collected south of the United States, an approximation of the cycle of day-lengths experienced by migrating bobolinks is constructed, which indicates that an "average" bobolink may experience in autumn only a few weeks of day-lengths less than about 12 hours.

4. Beginning October 2, when captive bobolinks were experiencing natural daylengths of about 12 hours 41 minutes, some were exposed to constant daily photoperiods of 12 hours for four weeks, others to 12-hour photoperiods for five or six weeks, after which the photoperiod was increased to 14 hours. Testicular recrudescence occurred in all. It is concluded that photorefractoriness can be overcome in this species by only a few weeks of exposure to photoperiods which in length are comparable to those it encounters in transequatorial migration but which are longer than those which maintain refractoriness in such temperate zone forms as Junco hyemalis and Zonotrichia albicollis. The number of shorter days required for the termination of refractoriness seems to be of the same general order of magnitude as for Junco and Zonotrichia.