Annual Rhythms of Gonadal Size, Migratory Disposition and Molt in Garden Warblers Sylvia borin Exposed in Winter to an Equatorial or a Southern Hemisphere Photoperiod

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Abstract
The Garden Warbler is a temperate-zone breeding bird that winters in tropical and southern Africa. To study the effects of internal timing processes and of photoperiodic conditions on the seasonal activities occurring before and after the breeding season, Garden Warblers were exposed from late September to late May to photoperiodic conditions experienced by free-living conspecifics wintering at the northern or the southern edge of the wintering area; i.e. the equator or 20.degree.S. Changes in body weight, migratory restlessness and gonadal size as well as the occurrence of molt were investigated. The birds exposed to a constant equatorial photoperiod carried out a complete prenuptial molt after autumn migratory restlessness. Subsequently gonadal development and spring migratory restlessness were initiated. These results indicate that photoperiodic changes are not necessarily required for the direct control of these activities. Photoperiodic conditions do, however, affect the timing of these processes. This was indicated by the performance of the warblers exposed to the 20.degree.S photoperiodic simulation under which most of the events investigated occurred earlier than in the birds held in the equatorial photoperiodic simulation. The acceleration was not sufficient, however to allow a full reproductive cycle during the southern hemisphere summer. In general, the results indicate that these long-distance migrants differ from all-year-round temperate-zone residents with regard to both the significance of endogenous timing factors and the photoperiodic conditions under which refractoriness can be terminated.