Abstract
The present study investigated (1) the photoperiodic threshold for an increase in FSH and LH secretion in male great tits (Parus major) from south Sweden and (2) seasonal changes in FSH secretion patterns in short day (SD; 8 L:16 D) and long day (LD; 20 L:4 D) exposed great tits. When fully photosensitive great tits were exposed to a simulated vernal increase in day lengths plasma levels of FSH started to rise at a shorter day length (10.5 h) than did LH (11 h). Once the photoperiodic threshold was exceeded FSH levels increased slowly, but continuously, and reached maximum levels when day lengths had reached 13–14 h, whereas LH levels rose to maximum levels more or less immediately. Great tits break photorefractoriness in September. Exposing sham-operated and castrated great tits to SD or LD in October and February resulted in clear seasonal differences in their FSH cycles. At both times the great tits responded to castration and exposure to long days. However, castration and photoperiodic FSH responses were not only more rapid in February, but also of a higher magnitude.
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