Little is known about the migration and movements of migratory tree-roosting bat species in North America, though anecdotal observations of migrating bats over the Atlantic Ocean have been reported since at least the 1890s. Aerial surveys and boat-based surveys of wildlife off the Atlantic Seaboard detected a possible diurnal migration event of eastern red bats (Lasiurus borealis) in September 2012. One bat was sighted approximately 44 km east of Rehoboth Beach, Delaware during a boat-based survey. Eleven additional bats were observed between 16.9 and 41.8 km east of New Jersey, Delaware, and Virginia in high definition video footage collected during digital aerial surveys. Observations were collected incidentally as part of a large baseline study of seabird, marine mammal, and sea turtle distributions and movements in the offshore environment. Digital survey methods also allowed for altitude estimation for several of these bats at >100 m above sea level. These observations provide new evidence of bat movements offshore, and offer insight into their flight heights above sea level and the times of day at which such migrations may occur.
Indirect evidence of the coastal migration of eastern red bats has been acknowledged in the scientific literature for over a century [2,3]. The seasonal appearance of eastern red bats in Bermuda, for instance, indicates their ability to travel long distances (>1000 km) over open ocean [10,11]. Fossil remains of red bats in Bermuda dating back to the middle Pleistocene suggest that this route over the Atlantic Ocean may be an established migratory pathway for this species . Several anecdotal accounts of red bats at sea provide further evidence that this species regularly travels over open ocean and along the outer continental shelf during migration (Table S1). Documentation of calm weather preceding and following bat encounters hundreds of miles offshore suggests that these encounters may be the result of normal migratory movements, rather than weather-driven deviations from land [13,14].
Tree bats on migration have recently encountered new threats during this important life stage. While wind energy development is largely viewed as an environmentally friendly energy alternative to fossil fuels, terrestrial wind turbines have been found to cause direct mortality to bats via collisions with the rotors, nacelles, and other infrastructure [7,8,15,16]. Recent post-construction monitoring studies at terrestrial facilities in the U.S. have found that the three northeastern migratory tree bat species constitute 75% of all bat fatalities at terrestrial commercial-scale wind facilities , with eastern red bats accounting for the greatest proportion of fatalities in the eastern U.S. [7,8].
Wind energy facilities have been operational in the offshore environment in Europe since 1991 . Despite evidence of bats migrating or foraging up to 80 km offshore in the North and Baltic seas [18,19], there have been no studies to date that have attempted to document collision mortality of bats with offshore wind turbines. However, several studies have stated that such mortality could occur, based on bat behavior offshore [19,20]. Given that eastern red bats are observed more frequently than any other bat species off the Atlantic Seaboard (Table S1), they may be most likely to interact with future offshore wind facilities in this region.