Spring migration of Indiana bats (Myotis sodalis) and what it means for the wind industry

The Indiana bat has been federally endangered for almost 50 years, yet there are still important information gaps in the species life history. How these bats migrate is a gap that we set out to fill. This research could also... [ view full abstract ]

The Indiana bat has been federally endangered for almost 50 years, yet there are still important information gaps in the species life history. How these bats migrate is a gap that we set out to fill. This research could also aid the wind industry in determining effective mitigation to reduce bat mortality at wind turbines. We hypothesized that bats would migrate north, require foraging stopovers during migration flight, and be negatively affected by inclement weather. We partnered with state and federal agencies in Tennessee and Indiana to learn where hibernating bats in each state were summering, with the intention of locating previously unknown maternity colonies. We radio-tagged bats from multiple hibernacula in April of each year from 2009 – 2016 and actively tracked them using aerial and ground tracking techniques. We collected individual location points throughout the night while the bats were active to determine the direction migrated, the duration and speed of nightly flights and the overall migration journey, data about foraging areas, and activity correlated with ambient weather. We documented connections to 16 maternity colonies in 6 states from 5 caves, 14 of which were newly discovered summer colonies. We found that there was no consistency in migration direction, i.e., they did not all migrate north. On average, it took bats 2.7±0.4 (SE) nights (1 – 5 nights) of active flight to arrive at their summer grounds. However, the average duration of the entire migration journey was 5.3±1.3 nights (1 – 16 nights). This discrepancy was due in part to the overall distance bats were traveling (x̅ = 162.5±24.1 km, range: 5.8 – 368.1 km) and the nightly speed (x̅ = 9.4±0.8 km/hr, range: 0.7 – 19.0 km/hr). However, the chief cause for extended layovers during the migration journey was due to cold temperatures and/or significant rain during the night. We never documented an active bat when the ambient temperature was below 10°C. Although bat activity in general has been observed at lower temperatures for other species and other regions, spring migrating Indiana bats may not have built up the physiological resources necessary to withstand colder temperatures and thus enter into torpor to conserve energy. Therefore, wind turbine curtailment could be reduced when ambient air temperatures drop below this minimum migration temperature. Over the years, we have tracked several migrating Indiana bats to the same maternity colonies, providing information about migration corridors for this species. Although there is some variation, it would be reasonable to map known straight-line connections between hibernacula and summer grounds and buffer them by 35 km (based on multiple bats tracked to the same summer grounds) in order to aid in siting decisions. Conservation measures could also be implemented for the habitat used by migrating Indiana bats. Landscape analysis of areas used by foraging bats could be utilized in conservation measures as well. Although we are starting to understand how Indiana bats move across the landscape, additional studies in other regions would help to supplement these data, as well as studies during fall migration.