Testing traditional assumptions about regional migration in bats

While some bats cover long distances during migration, moving thousands of kilometers, most migratory bats are considered regional migrants, thought to move relatively short distances (< 500 km) between hibernacula and maternity sites. However, behavior can vary considerably among species and our understanding of these movements has largely been limited to banding studies or detailed tracking of small numbers of bats by aircraft. Inferring population-wide behavior from small samples is difficult and can introduce bias. We tagged 108 Indiana bats (Myotis sodalis) in the Midwestern US and used a regional network of radiotelemetry receivers to study movement patterns. With this dataset, we tested the following traditional generalizations about regional migrants: (1) bats move away from hibernacula in spring in all directions with known maternity roosts, e.g., in a star-like pattern; (2) bats follow linear landscape features; (3) long-distance movements are uncommon; and (4) autumn migration comprises a single movement from summer maternity site to winter hibernaculum. In spring, bats left the hibernaculum immediately and primarily moved north despite available maternity roosts in all directions. We found no evidence that bats follow rivers, the predominant linear element in the landscape. Only six tagged bats traveled > 100 km, suggesting that longer-distance movements may be outliers. In autumn, only two bats visited multiple known hibernacula, and after swarming, some females moved > 100 km to areas without known hibernacula. Common generalizations about regional migrant movements may not be representative of population behavior and care should be taken with respect to management decisions based on those assumptions.