In order to increase the supply of renewable energy in the Netherlands, the Dutch government has supported the construction of a near-shore 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) and an effect study (Leopold et al. 2010, Krijgsveld et al. 2011) have been carried out to measure the impact of this single offshore wind farm on birds. 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. This study has been carried out by means of creating population models for different bird species. The effects of wind farms on these species, as the (estimated) number of victims, was fed into the population models. Most of these estimates were based on the radar measurements and field observations, including a ship-based monitoring program, gathered in and around OWEZ in 2007-2010.