1. Offshore flights of migratory landbirds are common but difficult to quantify due to the lack of offshore monitoring. Expanding offshore wind development is a potential risk to birds as overlap with migration corridors may increase.
2. Comparing terrestrial and offshore movements can reveal differences in migration that help target conservation efforts offshore. Understanding the timing, locations and numbers of offshore birds' movements is essential for identifying high-risk periods and areas for collision mitigation.
3. We used 16 coastal weather surveillance radars positioned along the western North Atlantic and Gulf of Mexico coasts of the United States to evaluate differences in passage, altitude and timing of migration between adjacent terrestrial and offshore habitats from 2014 to 2023. We trained an altitude extrapolation model to predict unobserved low altitude densities that the radar beam overshoots at increasing distances from the radar.
4. Migration traffic varied geographically and seasonally, with offshore migration traffic being lower in spring than in fall and lower than terrestrial migration year-round. Offshore migration showed shorter durations and fewer peak nights than terrestrial migration. Offshore flight altitudes were 13%–20% lower than over land. The tendency for offshore flights was higher in fall, especially near coastlines perpendicular to migratory directions.
5. Synthesis and applications. We identify regional patterns in offshore migration to inform wind energy siting decisions and operation. Migration of birds in the offshore environment at rotor-swept zones is common, and increasing wind energy operations will likely increase interaction between birds and rotors. The limited number of peak nights and shorter migration windows offshore increase opportunities for dynamic conservation. Curtailment targeting high-risk regions, seasons and nights will reduce operational downtime while protecting birds. Fall migration and areas downstream of coastlines perpendicular to migratory direction have a higher propensity for offshore migration, suggesting that spatial and temporal variation in migration intensity should be considered when balancing collision risks and operational costs.
