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Wind turbines don’t just wallop birds that collide with their spinning blades, they also shift where birds choose to fly and land. A new study shows that wind farms in the German North Sea have squeezed loons into a smaller resting spot along their spring migration route, which could make it harder for them to find food. Scientists suggest that as wind energy developments become more common, these subtler effects need to be considered.

Wind energy is the fastest growing energy source worldwide. While this reduces our dependence on fossil fuels, there are side effects. It has been estimated that 140,000 to 328,000 birds are killed by wind turbines each year in the United States alone. Birds of prey, like golden eagles, are hit hard by turbines because they use the same areas as wind farms—open expanses, such as farmlands, that are good for hunting and where strong winds act as highways for migration. Some birds, including sea ducks, have shifted their migration routes to avoid wind farms. That prevents collisions, but the altered flights can move the birds away from their food sources and waste precious energy if they have to fly farther.

Bettina Mendel at Kiel University in Germany and her colleagues set out to investigate the impact of wind farms in the German North Sea. More than 20,000 loons use this area to forage during their spring migration before returning to inland lakes to breed in the summer.

A marine protected area covering 3,135 square kilometers was established by the German government to preserve this loon habitat in 2005, but a cluster of offshore wind farms had already been approved inside and along the edge of the protected area. Today, 17 wind farms operate within the German North Sea, with five more under construction.

Loons don’t typically collide with wind turbines: they fly no more than 10 meters above sea level, whereas turbine blades typically spin 30 meters above the water. But Mendel and her colleagues searched for other effects.

They analyzed aerial and ship-based surveys from 2000 to 2017 to work out where the loons preferred to rest during their spring migration before and after the construction of four wind farms in 2014 and 2015. They found that the loons moved from two preferred areas (one where a wind farm had been built) to one hotspot smack in the middle of two farms. By avoiding wind farms, the birds lost the use of 275 square kilometers, or about nine percent, of the protected area. The authors note that squeezing loons into a smaller space could lead to more competition for food and, potentially, increased mortality.

Emma Kelsey, a wildlife biologist with the US Geological Survey in Santa Cruz, California, who has studied how offshore wind farms affect marine birds, calls the study “a big step forward” in understanding the indirect effects of wind farms. Those indirect effects, she notes, are less obvious but “potentially more impactful.” Studies like this one, Kelsey says, will help regulators consider where to build wind farms in the future.