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If adult starlings are kept on long days, they rapidly become refractory to long days (photorefractory). The recovery of the ability to respond to long days (photosensitivity) occurs after transfer to short days. Photosensitivity is associated with an increase in hypothalamic content of gonadotrophin-releasing hormone (GnRH), and, in castrated birds, with an increase in plasma LH. If photorefractory adult or juvenile starlings are transferred to short days, these events begin 3 and 4 weeks respectively after transfer. Starlings apparently hatch in a photorefractory state. However, if nestlings are hand-reared under short days, the increases in GnRH and LH begin at 6 and 7 weeks of age respectively. Is this 3-week delay, which corresponds to the time to reach adult body size, simply due to a slower response, or are nestlings incapable of responding to short days until fully grown? Castrated male nestling starlings were hand-reared under short days, under long days or under long days initially and transferred to short days at 2, 3, 6 or 9·5 weeks of age. The increase in plasma LH in birds transferred to short days at or before 3 weeks began at the same time as in birds kept on short days (7 weeks). In birds transferred at 3 weeks or later, the increase in LH began 4 weeks after transfer, irrespective of age. Intact female starlings showed similar responses. Birds kept on short days did not moult whereas birds transferred from long days to short days at 3 weeks did. These results suggest that nestlings are capable of perceiving daylength, but that the GnRH neurosecretory system does not respond to short days until birds are fully grown.

Journal of Endocrinology (1992) 132, 411–417

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