Abstract
Every year songbirds migrate in massive numbers across the North Sea. In the Dutch offshore waters, migration occurs in two major directions. One route is formed by birds migrating between breeding grounds on the mainland of northern Europe / Scandinavia and the UK. The other route is formed by birds migrating between northern Europe / Scandinavia and wintering grounds in southern Europe and Africa (Lack 1958-63, Myres 1964, Lensink et al. 2002). Although songbird migration largely takes place at higher altitudes, peaks in activity do also occur at lower altitudes, depending to some extent on weather conditions such as wind speed, wind direction and air temperature (Karlsson et al. 2011, Kemp et al. 2013). With the on-going development of offshore wind farms, migrating songbirds may collide with offshore wind turbines, potentially resulting in high mortality rates when bird fluxes at rotor height are high. A way to mitigate such mortality is by shutting down wind turbines during periods with high migration activity at rotor altitude. It is, however, largely unknown how often peaks of migration at rotor altitude occur. Here we show the occurrence of peaks in nocturnal migration activity of songbirds at rotor height at sea, as measured at the Dutch Offshore Wind farm Egmond aan Zee (OWEZ) between 2007 and 2011