Thyroxine induces photorefractoriness and stimulates prolactin secretion in European starlings (Sturnus vulgaris).

When subjected to 11 h daylengths starlings do not become photorefractory and remain reproductively mature indefinitely. Male birds kept in these conditions were treated for 5 weeks with thyroxine, administered in the drinking water at a concentration of 0.056 m mol/l. By the end of this period, their testes had fully regressed and they were then transferred to 18 h daylengths. No subsequent testicular development was noted and the birds were apparently photorefractory. Several other phenomena characteristic of the refractory state were also induced by the thyroxine treatment, namely change in bill colour from yellow to black, moulting of the flight feathers and a marked increase in plasma prolactin levels. These observations suggest that thyroid hormones may be responsible for termination of reproduction in this species.