During three seasons, we studied avoidance behaviour of migrating raptors when approaching an offshore wind farm in northern Baltic Sea 20 km from the coast.
From a substation 1.8 km west of the wind farm, we recorded macro, meso and micro avoidance behaviour of individual raptors approaching the wind farm, using a pre-defined protocol. We defined macro avoidance as when the raptor completely avoids entering the wind farm, meso avoidance as a significant change in altitude or direction before arrival and micro avoidance as a sudden change of flight when passing a turbine at close range.
In total, 466 migrating raptors representing 13 species were observed of which 73% representing 9 species showed macro, meso and/or micro avoidance behaviour.
Macro avoidance was recorded among ten of the species including 59% of red kites, 46% of common kestrels, 42% of sparrowhawks and 30% of honey buzzards. Three quarters of these raptors subsequently left the AOWF in a westerly direction, indicating that they returned to the mainland. The remaining birds flew either north or south parallel to the first row of turbines, suggesting they tried to navigate around the wind farm.
Our study demonstrated a barrier effect from the offshore wind farm influencing the migration of raptors by forcing some birds to use alternative and potentially more risky sea crossings. This may potentially affect survival and fitness of individuals and populations. Accordingly, we recommend that the location of important raptor migration routes is taken into consideration in siting of future offshore wind farms.
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