Drivers of spring migration phenology in Rocky Mountain elk.

By migrating, ungulates take advantage of cyclical fluctuations in resources, which allows them to persist at greater population numbers than they would in the absence of these seasonal movements. We sought to identify the drivers of spring elk (Cervus canadensis) migration and evaluate how well individuals were able to optimize access to forage prior to departure, while migrating, and upon arrival on summer range. Specifically, we investigated the timing and duration of spring migration in four Colorado elk herds to test how forage quality and snow dynamics pre-, during, and post-migration influenced elk departure from winter range and the length of time spent migrating. Our analyses revealed significant heterogeneity among and within herds. Overall, spatiotemporal dynamics in forage and snow emerged as critical drivers influencing migratory phenology, but the discrete covariates associated with forage and snow were herd-specific. We did not find marked sub-strategies wherein some herds or individuals optimized a specific component of migration, but rather found that elk varied in their ability to optimize access to forage at all times analyzed. Our findings suggest that elk exhibit a flexible response to environmental cues, adjusting migration timing and duration in accordance with local conditions. These results contribute to a deeper understanding of the mechanisms driving ungulate migration and emphasize the importance of considering dynamic environmental factors in studies of migration phenology. Further, they emphasize the importance of forage conditions not just on the migratory route, but also on winter and summer range. Additionally, they provide baseline knowledge of elk migrations in Colorado, useful information for ensuring the conservation and persistence of these migratory routes and the productivity they facilitate.