Circannual rhythms in starlings: response of castrated birds to 12-hr days depends on photoperiodic history.

The annual cycle of reproductive function in starlings (Sturnus vulgaris) is driven by seasonal changes in daylength and consists of a 6-month period of photosensitivity, culminating in maximal gonadal development, followed by a 6-month period of photorefractoriness. If male starlings are held under a constant photoperiod of 12 hr of light per day (LD 12:12), some individuals show circannual rhythms of testicular size. Do these rhythms consist of alternating periods of photosensitivity and photorefractoriness, and does the response depend on whether daylength is increased or decreased to 12 hr? Castrated starlings were transferred from natural short days in January (LD 9.5:14.5) to LD 12:12. Plasma LH decreased and then remained low for 2.5 years. Most birds molted, some twice. Other castrated birds were transferred from long days in August (LD 16:8) to LD 12:12. Plasma LH increased and then remained high for 15 months. None of these birds molted. Young starlings were exposed to LD 12:12 throughout incubation and thereafter. Plasma LH remained low until birds were 13 months old. All of these birds molted. These results demonstrate, first, that castrated starlings show no evidence of alternating periods of photosensitivity and photorefractoriness under LD 12:12; and second, that LD 12:12 can be perceived as a long day or a short day depending on photoperiodic history, and that young birds with no experience of any other daylength perceive LD 12:12 as a long day.