Photorefractoriness in European Starlings: Critical Daylength Is Not Affected by Photoperiodic History

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Abstract
Photosensitive ovariectomized starlings (Sturnus vulgaris) were transferred from short days (8L:16D) to long days (13L:11D) or to very long days (18L:6D). In birds transferred to 18L:6D, plasma LH levels decreased from maximal to minimal levels by 6 wk as photorefractoriness developed. In birds transferred to 13L:11D, the onset of photorefractoriness was slower, and LH reached minimal levels by 12 wk. Both groups of birds molted; those on 18L:6D began 7 wk after transfer from 8L: 16D, and those on 13L:11D began after 12 wk. Once molt had begun, it progressed at the same rate in both groups. After 24 wk, the birds on 18L:6D were subdivided into three groups; one was transferred to 8L:16D, another to 13L:11D, and the third was kept on 18L:6D. The birds on 13L:11D were also subdivided after 24 wk; one group was transferred to 18L:6D, another 8L:16D, and the third was kept on 13L:11D. In those birds transferred to 8L:16D, whether from 18L:6D or 13L:11D, plasma LH increased from minimal to maximal levels between 4 and 6 wk later, as photorefractoriness was terminated. In all other groups, plasma LH remained at minimal levels for the next 18 wk, indicating that 13L:11D is not perceived as a short day after exposure to 18L:6D. In those birds transferred from 13L:11D to 18L:6D, a second molt began 4 wk after transfer. These results provide further evidence that, in birds which become absolutely refractory, the overriding effect of long days is to switch the reproductive system off, while short days switch it on, and they also demonstrate that critical daylength does not change during exposure to long days. This is discussed in relation to birds whose breeding season is not terminated by absolute photorefractoriness.

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