- A new study looks at bird migration patterns over open ocean in an attempt to assess how much risk offshore wind turbines and other marine infrastructure might pose to them.
- The authors used radar data from U.S. coastal weather stations to find that hundreds of millions of birds migrate over tight windows of time in the spring and fall while flying at slightly lower elevations on average than over land.
- This puts a proportion of them at risk of being killed by wind turbines, but that risk could be mitigated with dynamic management that accounts for their patterns, according to the study.
- The Trump administration, in office since January, says it doesn’t support offshore wind development, but the research has long-term implications and could be used more immediately for mitigating the impact of offshore oil and gas projects.
When songbirds migrate through the Americas, they often cut through sections of the North Atlantic and Gulf of Mexico to save energy. A new study looks at their flying patterns in an attempt to assess how much risk offshore wind turbines and other marine infrastructure might pose to them.
The authors used radar data from U.S. coastal weather stations to find that hundreds of millions of birds migrate over tight windows of time in the Northern Hemisphere spring and fall while flying at slightly lower elevations on average than over land. This puts a proportion of them at risk of being killed by wind turbines, but that risk could be mitigated with dynamic management that takes into account their patterns, according to the study, which was published Sept. 16 in the Journal of Applied Ecology.
The Trump administration, in office since January, says it doesn’t support offshore wind development, but the authors still see their work as important to reducing the impacts of these projects on birds.
“We hope in the future, if offshore wind begins to materialize over time — I know there’s a bit of a pause on that right now — but even things like oil and gas and offshore flaring, [we’re] hoping that these numbers can be used to provide some guidance in terms of mitigation,” Shannon Curley, a movement ecologist and lead author of the study, told Mongabay. (Curley is an adjunct lecturer at Stony Brook University but undertook this work as part of a postdoctoral fellowship at the Cornell Lab of Ornithology at Cornell University; both universities are in New York state in the U.S.)
The migrating birds aren’t seabirds but mostly songbirds, which tend to migrate at night. Research had already been done on terrestrial bird migration, but this is one of the first large-scale, multi-site studies on offshore bird migration in the Americas.
“Radar is really good at picking [bird migration] up at these broad geographic scales, and then our hope was to extend this into the offshore habitat, which is what I got to do for this work,” Curley said.
The authors used nighttime radar data from 16 weather stations that are primarily run by the U.S. National Oceanic and Atmospheric Administration. They looked at data up to 75 kilometers (47 miles) from the stations for 2014-2023. One challenge they faced is that, partly due to the curvature of the Earth, the radar doesn’t gather data at the lowest altitudes as the distance from the station increases. To fill this gap, they used machine learning to train a model to estimate bird traffic at the lowest altitudes based on what was happening above.
The results showed that migration altitudes over water were 13-20% lower than those measured over land. That is, birds fly lower over water than over land. In fact, they found that 20-40% of bird traffic occurred at altitudes below 300 meters (984 feet), which is roughly the maximum height of most offshore turbine blades. While these low flyers weren’t in the majority, it still means millions of birds are at risk, indicating that management measures could be needed to prevent bird deaths from offshore wind.
Another finding could make such management seem slightly more achievable. Migration was more concentrated — it took place on fewer nights — over sea than over land. On average, 90% of migration activity occurred over 36 nights at sea, versus 43 nights on land; in the fall, the disparity was larger: 37 nights at sea versus 59 on land. (Bird numbers are higher in the fall because it’s after breeding season and there are simply more of them.)
The birds are picky about when they cross the water, Curley said.
“Migration is inherently dangerous, so I think what they’re doing is waiting for these optimum times where the winds are more cooperative for them to make these long overwater bouts,” she said.
That timing could bode well for interactions with offshore wind: If birds like to travel when it’s less windy, it wouldn’t necessarily be a big sacrifice for wind farm managers to turn their turbines off at those times.
The authors call for dynamic management of offshore wind that takes bird migration into account. They call for site-specific research and ultimately the development of offshore bird migration forecasts that managers can use. A forecasting tool called BirdCast already exists for terrestrial migration.
The study was funded by the U.S. Bureau of Ocean Energy Management under the Biden administration, which was in office for four years until January 2025 and championed offshore wind. (Around 10% of electricity in the U.S. was generated by wind power as of 2024. Only a tiny fraction of that was from offshore wind, but plans were underway for an offshore scaling-up during the Biden administration.)
Wind turbines are responsible for hundreds of thousands of bird deaths in the U.S. every year, but that’s only a small fraction of human-caused bird mortality. Domestic cats and non-turbine collisions are far more common.
In any case, this study can be used by managers to mitigate deaths not just from offshore wind but also from other offshore activities, including oil and gas projects, according to the study authors and Benjamin Van Doren, an assistant professor of natural resources and environmental sciences at the University of Illinois Urbana-Champaign, who wasn’t involved with the study.
“Songbirds migrating over inhospitable open water are already at increased risk because they cannot land to rest or refuel,” he told Mongabay in an email. “[I]t is therefore particularly important that offshore infrastructure minimize impact. Offshore infrastructure of all types continues to grow globally, making this study’s findings highly relevant regardless of current U.S. government priorities.
For now, there’s little data regarding bird deaths from offshore infrastructure or activities such as for oil and gas.
“It’s really hard to study offshore mortality,” Curley said. “If a bird collides with the structure, they’re immediately wind swept into the ocean. So it’s hard … especially when they’re migrating at night.
“Any time humans alter the environment, we put wildlife at risk for interacting,” she added.
Banner image: Birds flock in the foreground against wind turbines at the Spearville Wind Farm Sunday, Sept. 29, 2024, in Spearville, Kan. Image courtesy of AP Photo/Charlie Riedel.
Bright, unique colors can put tropical songbirds at greater risk
Citations:
Curley, S. R., Farnsworth, A., White, T. P., Shamoun‐Baranes, J., & Dokter, A. M. (2025). Differences between terrestrial and offshore bird migration: Implications for offshore wind energy. Journal of Applied Ecology, 62(10), 2800-2813. doi:10.1111/1365-2664.70158
Loss, S. R., Will, T., & Marra, P. P. (2015). Direct mortality of birds from anthropogenic causes. Annual Review of Ecology, Evolution, and Systematics, 46(1), 99-120. doi:10.1146/annurev-ecolsys-112414-054133
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