Monitoring of bird migration at marine wind farms has a short history, and unsurprisingly most studies have focused on the potential for collisions. Risk for population impacts may exist to soaring migrants such as raptors with K-strategic life-history characteristics.
Soaring migrants display strong dependence on thermals and updrafts and an affinity to land areas and islands during their migration, a behaviour that creates corridors where raptors move across narrow straits and sounds and are attracted to islands. Several migration corridors for soaring birds overlap with the development regions for marine wind farms in NW Europe. However, no empirical data have yet been available on avoidance or attraction rates and behavioural reactions of soaring migrants to marine wind farms. Based on a post-construction monitoring study, we show that all raptor species displayed a significant attraction behaviour towards a wind farm. The modified migratory behaviour was also significantly different from the behaviour at nearby reference sites. The attraction was inversely related to distance to the wind farm and was primarily recorded during periods of adverse wind conditions. The attraction behaviour suggests that migrating raptor species are far more at risk of colliding with wind turbines at sea than hitherto assessed.
Our investigations of behavioural responses of migrating raptors to a marine wind farm in the Baltic Sea provided, we believe, the first evidence of attraction of soaring bird species to marinewind farms as measured by migration directions. As the attraction behaviour is inversely related to the distance to the wind farm and is most significant during periods of head winds, it is likely that this behaviour is triggered by adverse weather conditions. Birds are attracted to offshore structures for various reasons [12–14]. However, an ‘island effect’ similar to the process that causes attraction of landbird species to small islands  is perhaps the most likely driver behind the attraction behaviour. The attraction leads to macro AR values, that are very different from those reported for waterbirds and seabirds, which typically indicate that more than 90% of the birds avoid marine wind farms . The AR values estimated in our study also show that behavioural reactions by soaring migrants at marine wind farms differ significantly from those recorded at land-based wind farms, where high AR values of several species of raptors have been reported . Behavioural details recorded at the Rødsand II marine wind farm further indicate low migration speed of raptors and complex movement patterns when they cross the wind farm [17,18].
The attraction of soaring migrants to marine wind farms will markedly enhance their risk of collision, as the migration heights of most species often overlap with the height of the rotor-swept zone . The migration height is however dependent on both wind conditions and visibility, with a higher proportion of birds flying at risk during adverse weather conditions . As many species of raptors and other soaring migrants have K-strategic life-history characteristics, the increased risk of added mortality at marine wind farms means that population impacts may occur unless proper planning of the development and siting of marine wind farms takes place in the future.