Seasonal changes in follicle-stimulating hormone in free-living great tits.

We describe the annual cycle in plasma levels of FSH in free-living male and female great tits from southwest Sweden. Both juvenile (here defined as first-time breeders or birds <1 year old) and adult great tits, of both sexes, showed clear annual cycles with three periods of elevated FSH levels: period from territorial establishment till end of breeding (mid-March to June), October (a time when the birds break photorefractoriness), and winter (January-February). Significant differences between ages and between sexes occurred only during March and April (period of territorial establishment and gonadal recrudescens). Male FSH levels increased significantly as early as between early February and early March. Levels continued to increase until mid-April when maximal values had been reached in paired males. Territorial, but unpaired, males had significantly lower plasma levels of FSH in mid-April than did territorial and paired males. After this, FSH levels did not change until levels had decreased to basal in early July. On average, females did not show a vernal increase in FSH levels until early/mid April. However, just as in males, female FSH levels were dependent upon whether she was paired or unpaired. Females having a mate in mid-April had significantly higher FSH levels than did single females. Contrary to the males, females showed a transitory FSH peak during egg-laying. Females showed no differences between other breeding stages, until basal levels were reached during incubation of the second clutch in July. Adult birds (of both sexes) had significantly higher plasma levels of FSH than juvenile birds of the same sex during the period March-April. Furthermore, during this period adult males continually had higher FSH levels than did adult females, and juvenile males continually had higher levels than did juvenile females. In all groups a second period of elevated FSH levels occurred during early October, a time when the great tits break photorefractoriness. All groups showed basal levels during November and December. By January all four groups had increased their circulating levels of FSH to moderately high. This elevated level was maintained during February and was at the same level as that observed in October.