As marine renewable energy applications are increasing in the UK, environmental and cumulative impacts and their assessments are receiving considerable attention. The uncertainty, particularly surrounding cumulative impacts, however remains high and is becoming a cause of delay in the consenting process. This study examines the sources of uncertainty in environmental impact assessments and provides recommendations as to how these may be reduced, using the example of birds and wind farms. In the assessment of environmental impacts, uncertainty not only enters through the availability of empirical data but also data collection and analysis methods, linguistics and policy frameworks. Identifying and quantifying sources of uncertainty in environmental and cumulative impact assessments is critical to facilitating the development of an environmentally sustainable offshore wind industry.