In order to increase the supply of renewable energy in the Netherlands, the Dutch government has supported the construction of a nearshore wind farm of 36 Vestas V90/3MW wind turbines 10-15 km off the coast of Egmond aan Zee, in the Netherlands (OWEZ, Offshore Wind farm Egmond aan Zee). This project served as a demonstration project to build up knowledge and experience with the construction and exploitation of large-scale offshore wind farms. In order to collect this knowledge, an extensive Monitoring and Evaluation Program (MEP- NSW) has been designed in which the economical, technical, ecological and social effects of the wind farm are gathered. Within this framework a baseline (Krijgsveld et al. 2005, Leopold et al. 2005) as well as an effect study (Leopold et al. 2010, Krijgsveld et al. 2011) have been carried out to measure the impact of the wind farm on birds. Those studies describe the impact of a single wind farm. In the present study we attempt, for the first time, to estimate the cumulative effects of multiple offshore wind farms in part of the North Sea on the population levels for a range of bird species. The offshore wind farm at Egmond aan Zee was the first offshore wind farm built in the Netherlands, with a second one completed one year later (but not studied as part of the OWEZ report). The Dutch government supports plans to build more turbines at sea in the coming years. As described in the following paragraphs a single wind farm will have certain effects on birds by means of collision, disturbance and/or barrier effects. Single wind farms might have a minor impact on the reproduction and survival (and thus population sizes) of birds as shown in several studies on single wind farms. Numerical impacts are mainly on a local scale by changes in distribution. The greater the effect, in terms of a decrease in reproduction and/or survival, the greater the impact will be on the population size. To this end, the construction of multiple wind farms at sea has the potential to reach the level above which survival and reproduction are significantly affected, which could potentially lead to a decrease in population levels at the wider (international) scale.