The common crane (Grus grus), like other soaring species of birds, uses a migration strategy to minimise the risk of wind drift over open sea, resulting in aggregation of birds along migratory corridors. With the planned large-scale development of offshore wind farms there is a need for an improved understanding of the potential interactions between cranes and the wind turbines as they cross the sea during migration. 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 between Sweden and Germany. The effect of weather conditions on the vertical distribution (i.e. flight altitudes) of the cranes was modelled using generalized additive mixed models. The results show that the flight altitude of common cranes crossing the basin strongly depends on the wind and visibility conditions. Both during the spring and autumn migration the cranes utilise thermal winds at the coast to soar, and frequently reach altitudes >300 m. Despite variability in wind and visibility conditions, the model predictions showed that the flight altitude descended towards the central offshore parts of the basin with a steeper descending trend during headwind and during poor visibility. The Arkona Basin is currently the focus of large-scale offshore wind farm development activities with a full built out of the region’s capacity for offshore wind projected to cover approximately 80% of the migration corridor. Our results indicate that 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.

The common crane (Grus grus), like other soaring species of birds, uses a migration strategy to minimise the risk of wind drift over open sea, resulting in aggregation of birds along migratory corridors. With the planned large-scale development of offshore wind farms there is a need for an improved understanding of the potential interactions between cranes and the wind turbines as they cross the sea during migration. 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 between Sweden and Germany. The effect of weather conditions on the vertical distribution (i.e. flight altitudes) of the cranes was modelled using generalized additive mixed models. The results show that the flight altitude of common cranes crossing the basin strongly depends on the wind and visibility conditions. Both during the spring and autumn migration the cranes utilise thermal winds at the coast to soar, and frequently reach altitudes >300 m. Despite variability in wind and visibility conditions, the model predictions showed that the flight altitude descended towards the central offshore parts of the basin with a steeper descending trend during headwind and during poor visibility. The Arkona Basin is currently the focus of large-scale offshore wind farm development activities with a full built out of the region’s capacity for offshore wind projected to cover approximately 80% of the migration corridor. Our results indicate that 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.