PHOTOPERIODIC CONTROL OF THE TERMINATION OF BREEDING AND THE INDUCTION OF MOLT IN-HOUSE SPARROWS PASSER-DOMESTICUS

Two photoperiodic mechanisms controlling gonadal regression in birds h ave been identified: absolute photorefractoriness, typical of species with short breeding seasons, where gonadal regression occurs spontaneo usly during long days, and relative photorefractoriness, where a decre ase in daylength is required to induce regression. An experiment was d esigned to test whether these simply represent extremes of one underly ing mechanism. Three groups of male House Sparrows Passer domesticus w ere transferred from a short photoperiod, 8 h of light: 16 h of darkne ss per day (8L:16D) to long photoperiods of either 18L:6D, 16L:8D or 1 3L:11D, Gonadal maturation rates were similar in all three groups; gon adal regression and moult began latest in the 13L:11D group, Four addi tional groups of sparrows were transferred from 8L:16D to 18L:6D and t hen transferred to either 13L: 11D or 16L:8D prior to, or shortly afte r, the onset of gonadal regression, The decrease in daylength prior to regression had no effect on the timing of regression but did advance the onset of moult, Decrease in daylength after the onset of regressio n increased the rate of regression and the rate of moult. Because a de crease in daylength did not affect the timing of regression, the data do not support the hypothesis that absolute and relative photorefracto riness represent extremes of a single underlying photoperiodic control mechanism, The adaptive significance of the effects of decreasing day length on the rate of regression and moult is discussed.