## 1 Introduction  
Collisions with wind turbines are an expanding conservation concern for bats [1; 2; 3]. In North America, non-hibernating, migratory \"tree bats\" (Genera: _Lasiurus_ and _Lasiomycteris_) are particularly susceptible to collisions and are often the majority bat group in post-construction carcass surveys at wind energy facilities [4; 5; 6; 7; 8]. The tree bat mortality rate at wind turbines appears to be highly correlated with the seasonal movements of these species [9; 10; 11; 12; 13; 14; 15] whereby collisions are generally elevated in spring and maximized in fall migration periods [5; 7; 16]. Increased mortality counts during migration may be attributable to space-use increase due to fall mating and migration, erratic juvenile dispersal behavior, and general attraction to turbines [13; 14; 17; 18]. North American tree bats are known to fly offshore with some regularity. This was first documented in anecdotal historical sightings from ocean vessels large distances off mainland coasts [19; 20; 21] and observations of tree bats on the island of Bermuda [22]. In the eastern North America, recent research has discovered high-flying tree bats 8.4-44 km from the main shoreline [23; 24]. The occurrence of tree bats offshore and along shorelines follows a similar seasonal activity pattern to wind turbine collisions--a general peak during spring and fall migration [23; 25; 26; 27; 28]. The reasons for this behavior remains unknown but some speculate that the coastline serves as a topographic reference for navigation [29] or that favorable wind conditions over open ocean may aid in long distance migration [30]. It is posited that the eastern shoreline acts as a topographic barrier, concentrating southward migrating tree bats along the coast during fall [10]. Wind energy in the eastern United States is expanding at an accelerating rate, particularly in the offshore sector [31; 32; 33]. To date, two offshore wind turbine operations exist in the eastern United States that account for <50 MW capacity [33]. However, an increasing number of offshore projects are now leased and in the beginning construction phases. It is projected that these projects will account for more than 20 GW of rated capacity [33], a 400-fold increase. Although projections indicate offshore wind facilities will likely be concentrated in the wind resource rich Northeast, some development is proposed off the mid-Atlantic coast along Virginia, Delaware, Maryland, and New Jersey [33]. The impact this rapid development will have on bats is unknown, however, risk is certainly non-zero particularly for tree bats as they are the most susceptible bat group to collision (particularly during migration) and are the only bat group consistently seen at offshore localities (again, particularly during migration). Onshore, extensive monitoring at wind facility sites post-construction have offered successful data driven conservation strategies to minimize bat mortality at turbines including, but not limited to, acoustic deterrents [34; 35; 36; 37; 38; 39; 40] and curtailment [41; 42; 43; 44; 45]. Curtailment is based on the knowledge that most bats generally avoid flying in overtly wind conditions, i.e., avoiding speeds generally above 5 m/s [44; 45]. Therefore, at low wind speeds below this (or other) threshold(s), turbine managers feather turbine blades, bringing rotor movement to a minimum, and thereby minimizing bat fatalities. There has been some success in the use of curtailment to reduce bat mortality while also minimizing financial loss [42; 44; 46] through the development of \"smart curtailment\" algorithms. These are typically model based, multivariate approaches whereby curtailment is triggered by the expectation of high bat activity or probability of presence [42; 47; 48]. Although these strategies may hold promise for offshore wind energy impacts, unlike terrestrial systems, the factors that influence occurrence (and therefore the parameter values necessary to predict risk metrics) of bats offshore are poorly known.