Last year there were over 700 wind turbines in the Baltic Sea, most of them in German and Danish waters, and according to the plans for wind energy production in the future decade, the area of wind farms will be increased to more than 27,000 square kilometers, Marko Mägi, a bird ecologist at the institute of ecology and geosciences of the University of Tartu and senior specialist at the Environmental Board (Keskonnaamet), wrote in his blog (link in Estonian).

Each of the wind farms covers a large area and has a significant ecological impact, especially on species' habitats. Therefore, offshore wind farm planning requires information on species occurrence, migration and habitat use, he said.

In addition to altering feeding and nesting habitats, wind turbines also obstruct bird migration. Due to the difficulty of obtaining relevant data, which requires accurate monitoring of migratory bird behaviors, the mortality rate in collisions with offshore wind turbines is only an estimate.

A more accurate depiction would necessitate a thorough comprehension of migration pace, direction and altitude, as well as hourly and seasonal variations. Scientists started to collect this information using GSP transmitters.

Due to large-scale and long-term population losses across the entire East Atlantic Flyway, the curlew is a near-threatened species. Even though breeding curlews avoid encounters with onshore wind farms, they are among the most endangered migrating birds due to the risk of colliding with onshore wind farms.

Between 2018 and 2021, the migration of the Eurasian curlew, whose population has dramatically declined, was monitored using GPS transmitters as the species passed offshore wind farms. The birds were tagged in four different countries: Germany (Wadden Sea), France (Atlantic coast), Estonia and Finland.

The data gathered from 51 individuals, including six birds that nested in Estonia, indicated that their migration route mainly traverses the Baltic Sea. When returning from wintering grounds in western Europe in the spring, a significant portion of the migration takes place over the sea and only 6 percent over land.

Specifically in the western portion of the Baltic Sea, the path of the the curlew runs through wind farms, and the majority of wind farms that have been already constructed are situated along the curlew's primary migration route.

The Eurasian curlew migrates typically at altitudes below 250 meters: lower over the sea than on land, and higher in the spring than in the fall. Consequently, a collision with a wind turbine, within the rotor's working radius, is highly possible. The blade of the tallest wind turbine at the moment reaches 188 meters; the lowest height is only 20 meters above sea level.

The researchers argued that as technology advances and new turbines become increasingly more potent, the blades of future turbines will reach 150 to 300 meters in height, while median of migration height was 60 meters in autumn and 150 meters in spring.

Overall, this suggests that between 65 and 75 percent of the Eurasian curlew's autumn migration occurs at altitudes where there is a danger of collision with a wind turbine.

In the spring, the figures are between 50 and 62 percent, and even lower, at 27 to 37 percent, if migration below the rotor's working radius is excluded.

The highest peak of the Eurasian curlew's spring migration occurs between April 16 and April 25, accounting for 59 percent of the spring migration.

From mid-June to mid-July is the peak of autumn migration. It is also known that the migratory Eurasian curlew is most active in the late evening, at night and early in the morning, when the bird flies at an average speed of 56 kilometers per hour and slightly faster at night than during the day.

In the same way, the researchers know that the spring migration occurs at exact same time and does not change over the years, because it is primarily determined by genetics. Therefore, they argue it is feasible to deactivate wind turbines during the peak migration period and reduce the risk of collisions. 

Nevertheless, the autumn migration is more dispersed in time, making bird-friendly management of wind turbines a difficult task.

The problem is also relevant to other shorebirds that use the Wadden Sea to refuel before migrating across the Baltic Sea to their north-eastern breeding grounds.

Most offshore wind farms are currently located within the 25 percent and 50 percent core home-range, in which curlews spent high proportions of their time migrating at turbine height, and a significant increase in this overlap can be expected if planned future wind farm scenarios are realized.

Sensitive sea areas for curlews are located particularly in the nearshore south-western Baltic Sea and the adjacent mainland, although the latter may be of less concern given the significantly higher flight altitudes across the land.

At the conclusion of the study the researches urge that particularly sensitive and condensed migration periods during spring should be urgently considered in relation to potential restrictions to turbine operation times.

Given the large areas set aside for offshore wind power generation in the Baltic Sea, the current study suggests that marine spatial planning for offshore wind farms must take into account the potential loss of bird biodiversity.

The research is published in Animal Conservation.

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