Autumn migratory departure is influenced by reproductive timing and weather in an Arctic passerine

Abstract

Numerous studies of spring migration have discovered that environmental conditions experienced on the wintering grounds and/or during vernal migration itself can have “carry-over effects” altering timing or success of the subsequent breeding season. Few studies have evaluated whether breeding and pre-basic molt have carry-over effects on autumn migration. The aim of this study was to test the expectations that (1) at broad temporal scales carry-over effects from breeding and molt constrain migratory departure but that (2) at finer temporal scales local weather will further refine this decision. We monitored nests of Gambel’s white-crowned sparrows breeding in the low Arctic in Alaska over three years and used radio-telemetry to track autumn migratory departure. We found that reproductive timing and weather parameters, but not molt timing, contributed to variation in autumn departure from the breeding site. Birds that terminated parental care late in the summer departed from the breeding grounds late relative to other birds. Birds were more likely to leave on nights without precipitation, when barometric pressure was increasing, and when ground level south winds were prominent. We also observed that, on average, birds departed 2.5 h after sunset and shifted the hour of departure as sunset advanced over the migration season. Our findings, in conjunction with observations of migration from earlier studies, raise the possibility that global climate change may be delaying autumn migratory departure in Gambel’s white-crowned sparrows breeding in Alaska.