MOORE, Ignacio T.; WINGFIELD, John C.; Univ. of Washington; Univ. of Washington: Reproductive cycles in equatorial populations of rufous-collared sparrows, Zonotrichia capensis

Seasonally breeding birds in northern latitudes are typically photoperiodic and use changes in daylength to time reproduction. The neuroendocrine system mediates seasonal reproduction associated with photoperiodism. In response to increasing daylength, birds become photostimulated, which is accompanied by a dramatic increase in hypothalamic GnRH content. During photostimulation, the gonads recrudesce and plasma testosterone levels, in males, increase to breeding baseline levels. This increase in testosterone is responsible for the seasonal development of the neural song control system. During the pre-breeding season, plasma testosterone levels peak in response to social stimuli. In addition, plasma testosterone levels will often increase in response to challenges from other males. The environmental cues and control mechanisms of seasonal reproduction have not been well investigated in tropical birds for which photoperiodic cues are often minimal. We have identified two populations of rufous-collared sparrows, Zonotrichia capensis, in Ecuador that are only 25 km apart, but have diametrically opposed rainfall patterns because they are on opposite slopes of the Andes. Both sites are on the equator and are at high elevation with limited seasonal environmental changes other than rainfall. In males, plasma levels of testosterone are higher during the breeding than pre-breeding season and male-male interactions do not have an effect on plasma testosterone levels. Pairs exhibit synchronous seasonal growth of the gonads during the pre-breeding season. The two populations appear to breed primarily during their respective dry seasons. The seasonality of the neural song control system and hypothalamic GnRH are currently being investigated in these two populations of birds.