Perfito, N*; Meddle, S.L.; Sharp, P.J.; Wingfield, J.C.: Seasonal gonadal recrudescence in song sparrows (Melospiza Melodia): response to temperature cues

Reproduction is timed so the likelihood of offspring survival is optimal, and for seasonal breeders often coincides with peaks in food abundance. Changing of the seasons is predictable at mid-latitudes, and spring is forecasted by lengthening days in winter. However, additional environmental cues (e.g. temperature, rainfall) give more precise information about when subsequent food peaks will occur. We tested the effects of temperature on the seasonal reproductive development of male song sparrows (Melospiza melodia morphna) from two populations: the coast of Western Washington (3 m) and the Cascade Mountains (1600 m). In the field, the timing of gonadal recrudescence can differ between the two populations by as much as two months, but in the lab under identical conditions, testes grow at the same rate. To investigate the role of temperature on the rate of gonadal growth, we captured birds from each population and assigned them to one of two treatment groups (2×2 design). We held birds in temperature controlled environmental chambers that either mimicked temperatures experienced in the mountains or on the coast. We increased daylength on a natural schedule in all groups, and measured testis volume, levels of circulating testosterone and prolactin, and song rates. Increasing daylength stimulated gonadal growth in all groups. Only in the mountain birds did colder, montane temperatures produce slowed rates of gonadal growth, delaying the onset of growth by one month when compared to all other groups. Since temperature changes more markedly during the early winter months in the mountains than on the coast, increasing temperature may be a more relevant cue in timing of reproduction in the mountain population.