The Pulse of the Planet: Measuring and Interpreting Phenology of Avian Migration

Abstract

Changing phenology of bird migration has become a flagship example of the biological impacts of climate change. Bird migration phenology data come from a limited number of time series in idiosyncratic locations. Improved understanding of these relationships requires new data collected in a standardized method that spans spatial and temporal scales. We used weather surveillance radar data and eBird data to show that it is feasible to measure bird migration phenology from local to regional scales and that these data provide geographically informative and temporally consistent patterns of migration phenology. We examine both a single species (Purple Martin) case and widespread nocturnal songbird migration. We also analyzed patterns in potential environmental cues that migrants might use to adjust their phenology en route. These analyses suggest that temperature does form a thermal wave that is a plausible cue for adjusting migration timing, but that the Normalized Difference Vegetation Index (NDVI) as a measure of the start-of-spring does not produce a green wave that would be informative to migrants for gauging seasonal phenology. This result calls into question the use of vegetation indices to understand how migrants adjust their timing en route.