Flight Altitude of Common Cranes ( <i>Grus grus</i> ) Crossing the Arkona Basin (Baltic Sea): Implications for Offshore Wind Farm Development

With the planned large-scale development of offshore wind farms, there is a need for an improved understanding of the potential future interactions between migrating common cranes (<i>Grus grus</i>) and the wind turbines as they cross areas of open sea during migration. The Arkona Basin is currently the focus of large-scale offshore wind farm development activities, with full development of the region's capacity for offshore wind projected to cover approximately 80% of the migration corridor. By using laser rangefinder tracking and GPS-tagged crane individuals, we studied the vertical flight behaviour in relation to weather conditions as they cross the Arkona Basin in the Baltic Sea between Sweden and Germany. The effect of weather conditions on the vertical distribution (i.e., flight altitudes) of the cranes was modelled using generalised additive mixed models. The results show that the flight altitude of common cranes crossing the basin strongly depends on the wind and clearness conditions. Both during the spring and autumn migration, the cranes utilise thermal winds at the coast to soar and frequently reach altitudes > 300 m. Yet, the model predictions showed that the flight altitude descended towards the central offshore parts of the basin targeted for offshore wind farm development, with a steeper descending trend and flight altitudes at rotor height during crosswind and headwind conditions and during poor and moderate clearness (< 60%). Our results indicate that, in combination with their low level of macro avoidance, the overall collision risk of migrating cranes will depend on the frequency of adverse conditions, which cause the birds to fly at rotor height over the wind development zone. Implementation of efficient mitigation measures (e.g., turbine curtailment) to minimise the risk of collision with the future large-scale wind turbine installations in the region is obviously a conservation priority.