Environmental and anthropogenic correlates of seasonal migrations in the Dinaric-Pindos brown bear population

Seasonal migrations (i.e. seasonal round-trips between disjunct areas) have been rarely documented for large carnivores. The Dinaric-Pindos brown bear (Ursus arctos) population is the third largest in Europe, but little information is currently available on individual movement patterns. We studied movement patterns by 12 GPS-collared adult and subadult bears in Croatia and Bosnia Herzegovina during 2004–2017, including migratory movements by some instrumented bears. To investigate environmental correlates of migrations, we first used the canonical Outlying Mean Index analysis, identifying habitat descriptors of summer and fall ranges, and then applied mixed-effects logistic regression to quantify variation in habitat use between them. Thirty-seven per cent 37% of the bears (7 bear-years) migrated during hyperphagia (i.e. partial migration), and seasonal migration was also facultative, as it occurred only during mast (i.e. beechnut) poor years. Migrating bears entered migration during early fall (median = 25 Sept) and returned to their pre-migratory ranges after about 7 weeks (median = 18 Nov). Net distances between pre-migratory (summer) and post-migratory (fall) averaged (±sd) 26.5 ± 9.7 km, with a maximum distance of 38.8 km, corresponding to actual distances travelled of 61.1 ± 21.5 km. Summer ranges from which bears migrated were best described by proximity to supplemental feeding sites and mixed forests, whereas fall ranges reached by migrants were differentiated by lower elevations, and a higher share of deciduous forest, grasslands, forest edges and shrublands. Relative to pre-migratory ranges, bears in post-migratory ones increased their distance to anthropogenic features and showed higher use of cover types expectedly richer in berries and other fleshy fruits. Although we lack any causative evidence, we speculate migration in this bear population is triggered during poor beechnut years by increased social despotic interference at supplemental feeding sites that elicits redistribution of subordinate bears.