Meeting Abstract
Migration in birds often results in individuals dispersing over large geographic areas after the breeding season. Here we asked if within population variation in wintering latitude, conveyed via photoperiod, contributes to variation in timing of the seasonal activation of the reproductive axis. In response to increasing photoperiod in winter and spring, the avian reproductive axis becomes stimulated, resulting in a transition to reproduction. In late summer we captured male song sparrows (Melospiza melodia) from a breeding population in Indiana, USA (39.16°N, 86.53°W) and assigned them to one of two indoor aviaries under the natural photoperiod of the site of capture. In mid-October, when song sparrows migrate south, we shifted the photoperiod of one aviary to the natural photoperiod at the southern extent of their wintering range (27.95°N, 82.46°W), simulating autumn migration. From mid-October to mid-March, the photoperiods in both aviaries were adjusted to follow the natural seasonal progression of day length at the two latitudes. In mid-March, when song sparrows migrate back to their northern breeding grounds, we shifted the photoperiod of the migrant treatment back to the natural photoperiod at their breeding grounds, simulating spring migration. Once a week, beginning in February, we measured testosterone synthesis capacity using a gonadotropin-releasing-hormone (GnRH) challenge. Despite experiencing longer photoperiods during the winter, birds in the migrant treatment had lower testosterone in spring compared to birds that were exposed to the shorter northern photoperiods. Our results indicate that within population variation in migration distance contributes to variation in readiness to breed in early spring.
