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Seasonal changes in gonad weights, concentrations of plasma luteinizing hormone (LH), prolactin, thyroxine (T4) and triiodothyronine (T3), and 5β reductase activity in the neuroendocrine tissues were measured in the Grey partridge (Perdix perdix) in northern Italy. Seasonal changes in the birds' photoperiodic responses were investigated by transferring groups from natural to continuous lighting during most months of the year, and measuring the effect on gonadal weight and plasma LH. The role of increasing short daylengths in stimulating reproductive function in late winter was assessed by measuring testes weight and plasma LH in birds maintained on a fixed short daylength (9 h light: 15 h darkness) during January and February.

Egg laying occurred between mid-April and the end of June, coinciding with seasonally high concentrations of plasma LH and prolactin in both sexes and low plasma T3 in the females. In July, the gonads began to regress and plasma LH levels decreased but plasma prolactin levels remained elevated, falling during August and September. No changes in plasma T3 and T4 were seen in July. In the hyperstriatum (part of the forebrain) and anterior pituitary gland, 5β reductase activity was higher in July than in November, February or May.

Plasma LH levels increased after transfer to continuous light between October and April but did not increase after transfer to continuous light between June and September. The increase in plasma LH seen after transfer to continuous light in January, February, March and April was associated with an increase in gonadal size. In contrast, gonadal size did not increase, or did not increase greatly, after transfer to continuous light in October, November and December. Plasma LH levels increased and the testes grew in January and February, irrespective of whether the birds were exposed to natural lighting or to a fixed short daylength.

The major conclusions were that in the Grey partridge: 1, seasonal breeding is terminated by the development of long-day refractoriness; 2, the development of long-day refractoriness is associated with an increase in plasma prolactin and no change in plasma thyroid hormones; 3, increased 5β reductive activity in the hyperstriatum of long-day refractory birds may be related to the disappearance of sexual behaviour; and 4, the growth of the testes in late winter is independent of increasing daylength.