Circannual Rhythms in Starlings: Response of Castrated Birds to 12-Hr Days Depends on Photoperiodic History

Abstract

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?

Keywords

starling circannual rhythm photoperiodism photorefractoriness luteinizing hormone molt castration

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