Disrupted Skies: How Offshore Wind Farms Alter Flight Behavior of Breeding Seabirds

Offshore wind farms are expanding rapidly as part of global climate mitigation efforts, but their effects on seabird movement behavior remain incompletely understood. While collision risk has received substantial attention, less is known about how turbines may alter flight routes through evasive behavior and meso-avoidance, particularly near breeding colonies where repeated commuting flights may accumulate energetic costs. We investigated flight responses of breeding Bridled Terns (Onychoprion anaethetus) to offshore wind turbines near their colony using high-resolution satellite tracking data collected at 1-s intervals and lower-resolution data collected at 1-h intervals. We quantified within-trajectory flight traits, including mean redirection, number of turns, flight speed, and flight altitude, in relation to turbine exposure. We assessed avoidance using both proximity-based and direction-sensitive metrics. At the near-colony scale, we tested whether flight behavior changed with increasing alignment between the trajectory bearing and turbine bearing from the colony. At the broader breeding-range scale, we tested whether behavior differed inside and outside wind farms or with distance to turbines, while accounting for colony distance, wind, and landscape variables. Bridled Terns showed increased mean redirection and lower flight altitude when trajectories were more closely aligned with turbine directions from the colony, suggesting localized route alteration in obstacle-facing directions. However, flight behavior was not significantly associated with turbine proximity, nor did it differ significantly inside and outside wind farms. These findings suggest that offshore wind farms may influence seabird movement through localized, direction-dependent route alteration rather than simple distance-dependent responses, highlighting the value of movement-context metrics and within-trajectory traits in wind farm impact assessments.