Photoperiodic regulation of the male house sparrow song control system: Gonadal dependent and independent mechanisms

The primary and secondary sexual characteristics of many species of passeri ne birds undergo dramatic seasonal variation in response to the change in t he length of photoperiod. Among the many physiological processes that under go seasonal changes, bird song and the song control system underlying it un dergo similar seasonal variation in size and function. The mechanisms of th is seasonal variation are largely unknown but are at least partially due to steroidal action from the gonads. The present study determined the relativ e roles played by the gonads and the photoperiodic timing system that contr ols gonadal development on song control nuclei in the brain of the male hou se sparrow Passer domesticus. Sparrows maintained in short photoperiods (SD ) possessed small regressed testes. Transfer to long photoperiods (LD) for 6 weeks evoked a dramatic increase in testes size, but, after 20 weeks unde r the same conditions (LDLD), testes completely collapsed. Song control nuc lei HVC and RA were smaller in SD than in LD but regressed only moderately in LDLD. Castration of sparrows in SD reduced the amplitude of the seasonal variation but did not completely abolish it. The data support the view tha t the song control system of the house sparrow is regulated by the photoper iodic timing system independently of gonadal influence, but that the gonads augment seasonal regulation of song, presumably via steroidal hormone secr etion. (C) 2000 Academic Press.