Many temperate-climate bats migrate tens to hundreds of kilometers from hibernacula to summer habitat each spring and in the opposite direction each fall. Understanding the timing of migration can help reduce the risk of disturbance via anthropogenic activities, guide effective management, and determine future impacts of a changing climate. We examined the influence of weather and day of year on the arrival and departure of Indiana bats (Myotis sodalis) monitored at a summer maternity colony site in central Indiana from 1998 to 2014, using emergence count data to track the timing of bat presence during the spring and fall migration seasons. We used an information theoretic approach to compare 23 models that predicted bat presence as a function of weather, climate, and lunar illumination; these models predicted arrival of the first Indiana bat and the first observation of a colony in spring, and the last observation of a colony and the last bat observed in fall. Bats embarked on spring and fall migrations to the maternity colony area at approximately the same time each year (arriving ∼3 April and departing ∼7 October) and variation was accounted for by changes in weather. Spring arrival and colony formation were predicted by higher temperatures ( = 22.5°C for colony formation) and precipitation and lower wind speeds, whereas lower temperatures ( = 25.9°C for colony breakup) and precipitation and higher wind predicted colony breakup and departure in fall. Spring migration coincided with periods of increased winds and, thus, we advocate for higher cut-in speeds for wind turbines during bat migration seasons. Resource managers should consider the entire time that bats are on the summer landscape when defining regulations and implementing conservation measures.
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Vol. 98 • No. 5