Partial migration and philopatry of Yellowstone pronghorn

Spring migration may facilitate survival and recruitment in mammals by reducing predation risk and increasing access to higher-quality forage. The Yellowstone pronghorn Antilocapra americana population (<250 animals) retains one of only two pronghorn migrations remaining in the greater Yellowstone region of the western United States. We used 5743 telemetry locations of 44 adult, female pronghorn during June 1999–August 2005 to determine migration patterns, seasonal distributions, and individual fidelity to migratory strategies. Yellowstone pronghorn were partially migratory, with >70% of the pronghorn population migrating 15–50 km to 4 contiguous summering areas and <30% remaining year-round on the winter range. Most radio-collared pronghorn showed fidelity across years to their migration strategy and summer use area, but approximately 20% migrated in some years, but not others. This behavioral flexibility is consistent with the hypothesis that migration in Yellowstone pronghorn is a conditional strategy and likely contributed to dynamic and rapid changes in the proportion of migrants from 80% to 20% and back to 70% during 1967–2005. All migrant pronghorn traveled 10 km over a topographic bottleneck (Mt. Everts) separating the winter and summer ranges, primarily using grassland–sagebrush pathways through conifer forest. We recommend continued protection of this corridor because increased mortality and a decreasing proportion of migrants may be as important a threat to the persistence of partially migratory populations as habitat fragmentation, especially when local resources for non-migrants are inadequate to sustain the entire population.