A large proportion of nocturnal bird migration in Europe runs across the North and Baltic Seas.
It is generally assumed that the increasing number of offshore wind farms constitutes a growing potential collision risk for migratory birds and that the number of collision victims increases with increasing migration intensity. However, due to a lack of suitable recording methods at sea, there is currently little empirical data available on this subject.
Main objective of the study was to quantify the collision risk of migratory birds and to check whether there is a strong positive relationship between collision rate and migration intensity.
The investigations were carried out using different methods in a coastal onshore wind farm. Next to radar surveys (BirdScan MR 1, Swiss Birdradar Solution AG), a combination of AI-based stereo cameras (AVES Offshore HPC, ProTecBird GmbH) was used to accurately monitor migration activity and bird flight movements in the rotor swept area. In addition, the number of collision victims was determined in regular on-site inspections.
It was found that even in nights with high migration intensity, the probability of collisions did not increase. Thus, no strong positive correlation between high migration intensity and the probability of bird collisions could be confirmed.
The number of flights through the rotor-swept area during turbine operation was generally very low. At night, an average of one flight through rotor-swept area per 132 operating hours was recorded, if the rotor speed was two revolutions per minute or more.
It was particularly noteworthy that 20 times more birds crossed the rotor area of idle wind turbines than during operation. This indicates a reduced avoidance reaction of birds towards idle wind turbines.
It was found that more than 99 % of all birds avoided the rotors both during the day and at night. The estimated total collision risk for birds flying through the wind farm area was no higher than 0.004 %.
The results question previous assumptions on the collision risk. The transferability of the study to the offshore sector needs to be examined in more detail in future studies.
The study is available for download here:
