Control of testicular cycles in the song sparrow, Melospiza melodia melodia: interaction of photoperiod and an endogenous program?

Adult male song sparrows, Melospiza melodia melodia, underwent testicular development accompanied by increases in plasma levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) regardless of photoperiods tested. Although gonadal development progressed rapidly in birds exposed to long days, males subjected to short day lengths also showed testicular growth between 90 and 200 days. Onset of photorefractoriness during exposure to continual long days was highly variable among individuals. Short days were not required to regain photosensitivity, and thus spontaneous testicular recrudescence occurred in birds held on long days for prolonged periods. However, cycles of testicular mass, length of cloacal protuberance, and prebasic-type molt in birds held on continuous long days became dissociated and were erratic. It males held on short days were exposed to 1 long day (18L 6D) and then returned to short days, a rapid increase in testicular mass and plasma levels of LH and FSH followed. The effect of 1 long day was greater if males had been exposed to short days for 2 months or longer. Thus, although short days were not absolutely required for dissipating photorefractoriness, they apparently accelerate the process. Female song sparrows showed a similar response to 1 long day. Input from changing day length was also important for triggering testicular regression. Male song sparrows held on 16L 8D for 10 months showed no signs of prebasic molt or testicular regression. When half of the males in this group were then exposed to 14L 10D (a shorter day length but still photostimulatory in spring), there were rapid declines in testis mass and FSH levels. These results are consistent with the phenomenon of "relative photorefractoriness," in which decreased day length results in gonadal regression despite the fact that similar day lengths would have been photostimulatory earlier in spring. It is suggested that song sparrows have "programs" of gonadal activity and molt that are not usually expressed in captivity as part of a complete cycle. The annual change in day length may act to drive the cycle, as continual environmental input for entraining endogenous programs to exactly 1 year and for scheduling the correct temporal sequence of events. Such a combination of continual environmental input and endogenous rhythms may be widespread.