Male European starlings(Sturnus vulgaris) were held for three consecutive photoperiod oscillations (ahemeral years) composed of 30-h day lengths, i.e., the “daily” light and dark each lasted three hours longer than under the natural daily photoperiod at latitude 38°N. These starlings had no gonad metamorphosis during the 45 actual months necessary to complete the three ahemeral photoperiod oscillations; nor did subsequent exposure to continuous illumination elicit gonad response. It is concluded that the daily duration of light and darkness (although certainly operant in controlling starling sexual cycles under temperate-zone photoperiod oscillations) is not the critical factor establishing a sexual cycle under the ahemeral regimen. Rather, it appears that this species must experience a daily duration of light of 12 hours or less (a definitive photoperiodic threshold) before photo-induction of a sexual cycle is possible.
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Schwab, R.G. Ahemeral light regimens test the photoperiodic threshold of the european starling(Sturnus vulgaris) . Int J Biometeorol 24, 83–88 (1980). https://doi.org/10.1007/BF02245545
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