During the winter months, large flocks of sea ducks live in the waters around the islands of Fehmarn and Lolland. Feeding on mussels and crustaceans, sea ducks lead hard lives. Outside the breeding season, they inhabit the open sea where they are exposed to extreme weather conditions. They dive down to the seabed at depths of 30 meters or more to find hard-shell mussels.

In terms of numbers, the most important species in the Fehmarnbelt are eider of which there are currently some 250,000 wintering birds together with 16,000 long-tailed ducks and 35,000 common scoters. For all three species, the winter population in Fehmarnbelt accounts for a large proportion of the total population.

Bird count
Studies of sea ducks play a special role in connection with the EIA report. Populations and habitats are identified from vessels and by air. Of particular interest is the sea duck’s way of life which is being investigated in order to identify the potential consequences of changes to the local environment caused by a bridge or tunnel.

A bridge may, for instance, impede the exchange between important feeding grounds while a tunnel could impact on the food chain as a result of changes to the seabed and sediment spread. Over two winter periods, therefore, eider, common scoters and long-tailed ducks were caught and tagged with small transmitters which allowed the birds' location, movements and diving behaviour to be monitored.

Satellites and radio transmitters
Two different methods are employed for the studies: satellite transmitters and radio telemetry transmitters. By identifying the birds' position by satellite, their migration can be monitored over very long distances. This enables movements between different wintering areas in the Baltic Sea and migratory movements to the breeding areas to be monitored.

Programmed to transmit intermittently in order to save battery life, the satellite transmitters, however, only record a few positions a day. To register the birds' daily movements and activity, small radio telemetry transmitters are used. Signals can be received within a few kilometres by hand-held antennas. Since signals cease when the ducks dive for food, it is possible to identify precisely where the birds search for their food.