Abstract
Bird and bat fatalities increase with wind energy expansion and the only effective fatality‐reduction measure has been operational curtailment, which has been documented for bats but not for birds. We performed opportune before‐after, control‐impact (BACI) experiments of curtailment effects on bird and bat fatalities and nocturnal passage rates during fall migration at 2 wind projects, where 1 continued operating and the other shut down from peak migration to the study's end (study 1). We also performed BACI experiments during a 3‐year study of curtailment and operational effects on bird fatalities among wind turbines of varying operational status (study 2). In study 1, wind turbine curtailment significantly reduced near‐misses and rotor‐disrupted flights of bats, and it significantly reduced fatalities of bats but not of birds. In study 2, converting wind turbines from inoperable to operable status did not significantly increase bird fatalities, and bird species of hole or sheltered‐ledge nesters or roosters on human‐made structures died in substantial numbers at vacant towers. Of bird species represented by fatalities in study 2, 79% were found at inoperable wind turbines. Because the migration season is relatively brief, seasonal curtailment would greatly reduce bat fatalities for a slight loss in annual energy generation, but it might not benefit many bird species.