Rapid shifts in migration routes and breeding latitude in North American bluebirds

Spatial and temporal shifts in the migratory patterns of birds have become more frequent as climate change and habitat alteration continue to impact ecosystems and the species dependent on them. In this study, we used eBird community science data collected over ten years to examine potential changes in the migratory patterns of three North American bluebird species: eastern (Sialia sialis), western (Sialia mexicana), and mountain (Sialia currucoides) bluebirds. Community science datasets such as those provided through eBird are a valuable tool for examining population-level processes, as such data are often costly and time-consuming to collect through other approaches (e.g., directly tracking individuals). Using generalized additive models, we produced smoothed migration paths for all three species over each season from 2009 to 2018. We asked whether there were changes over this 10-year period in the timing of spring and fall migration and migration speed, and the population centroids during breeding and migration. In contrast to many species that are experiencing poleward shifts in their distributions, the population centroids during the breeding period of all three bluebird species appear to have shifted southward over the past decade. Perhaps most surprisingly, we also detected strong longitudinal shifts in the population centroids during migration in eastern and western bluebirds, with both species shifting toward the center of the continent. Despite these changes in migratory routes and breeding distributions, we detected no change in the migratory timing or speed of any of the species. Our analysis indicates that bluebirds are rapidly altering the pattern of their migration, likely in response to changing environmental conditions, but not always in the direction predicted.