Photophase and Testicular Recrudescence in European Starlings

In an elaborate sequence of experiments, Bissonnette (1930, 1931, 1937) demonstrated a positive correlation between testicular growth in European Starlings (Sturnus vulgaris) and seasonally increasing photoperiods occurring during the winter and spring months in north temperate zone latitudes. These results eventually led to the classification of the starling as a primary photoperiodic species (Farner and Lewis 1971). Berger (1947) reported that spontaneous testicular growth does not occur during the autumn months in temperate zone latitudes because natural photoperiods at this time are too short. Berger also documented that the refractoriness of the reproductive systems to long photoperiods immediately following a testicular cycle is naturally dispelled by late November. In a later study, Berger (1953) induced partial reproductive development in nonrefractory starlings held under stable photophases as short as 8.5 h per day. However, 8.5-h photoperiods did not allow complete testicular development to the spermatozoa-producing stage during the 20 weeks of treatment. Recent data suggest that classifying the European Starling as a primary photoperiodic species may be somewhat misleading. (1) Although seasonal testicular metamorphosis in starlings under natural temperate zone photoperiods is correlated with increasing daily photophase durations, such photoperiod increases are unnecessary. Adult postinvolution starlings held under fixed photoperiods ranging from 1 to 11 h subsequently reach histologically verified reproductive condition (an abundance of spermatozoa in the seminiferous tubules) despite the absence of daily photostimula-’ tion. (2) Photorefractory starlings chronically exposed to LD 10.5:13.5 and LD 11: 13 achieve and maintain reproductive condition for at least 15 months (Schwab 1970). (3) Prolonged treatment under a semi-equatorial photoperiod (chronic LD 12:12) results in spontaneous testicular growth-involution cycles with an average 9.5-month interval between cycles (Schwab 1971, Rutledge 1974). The present study was conducted in an attempt to refine our understanding of the photosexual response system in European Starlings, particularly the ways in which starlings appear to differ from many other photoperiodic avian species.