Since 2005, the Research Institute for Nature and Forest (INBO) performs monthly BACI-designed surveys to study seabird displacement following the construction of offshore wind farms in the Belgian part of the North Sea. For the first time since its completion in 2013 we report our findings for the C-Power wind farm at the Thorntonbank, and we also give an update of the results for the Bligh Bank wind farm after five years of post-impact monitoring.Compared to earlier reports and publications, we introduced some improvements in our modelling strategy. To correct for decreasing detectability with distance, the seabird numbers observed were now distance-corrected, and by allowing the detection functions to vary with wind force or wave height, temporal variation due to observation conditions was further reduced. We also included a fishery factor in the model, allowing to correct for the presence of beam trawlers in the vicinity of our survey tracks. As expected, this factor often explained a significant part of the variation in the counted numbers of gulls and northern fulmars.Based on the resulting impact models, we found significant avoidance by northern gannet and common guillemot at both sites. Common guillemot decreased in densities by 68% and 75% at the Thorntonbank and Bligh Bank respectively, and northern gannet by 99% and 82%. Razorbill decreased in numbers at the two sites, this decrease being significant at the Bligh Bank only (67%). Both sites attracted great black-backed gulls, this species having increased in numbers significantly by a factor 6.4 and 3.6 at the Thorntonbank and Bligh Bank respectively. The previously reported attraction effects of lesser black-backed gull and herring gull at the Bligh Bank were confirmed after two more years of monitoring, but no such effect was observed at the Thorntonbank. Finally, Sandwich tern appeared to be attracted to the offshore wind farm at the Thorntonbank, this effect being significant only for the buffer zone. This is in line with the results for the ...