Photorefractoriness in European starlings (Sturnus vulgaris) is not dependent upon the long-day-induced rise in plasma thyroxine.

Transfer of intact, photosensitive starlings from short to long days causes an increase in plasma thyroxine and gonadal maturation and later induces photorefractoriness. Thyroidectomy of starlings prevents the induction of photorefractoriness. This study investigated whether the long-day-induced increase in plasma thyroxine is necessary for the induction of photorefractoriness. Photosensitive starlings were thyroidectomised, given thyroxine in their drinking water at concentrations that result in plasma thyroxine at short-day physiological concentrations or lower, and transferred to long days. Plasma thyroxine and prolactin, gonadal size, and moult were monitored. The group with short-day concentrations of plasma thyroxine became photorefractory at the same time as intact controls transferred to long days. The other groups, with lower plasma thyroxine, also became photorefractory, but the onset of photorefractoriness was delayed. The increase in plasma prolactin following photostimulation was proportional to plasma thyroxine concentrations. The onset of moult was also related to plasma thyroxine. We conclude that the long-day concentrations of plasma thyroxine observed in the plasma of intact starlings are not necessary for the induction of photorefractoriness. This suggests that thyroxine acts as a permissive factor rather than actively driving the photorefractory process.