Abstract
Offshore wind (OSW) energy is rapidly expanding worldwide as countries aim to decarbonize their electrical grids. However, as OSW scales up to meet increasing demand, concerns about cumulative environmental effects have mounted. Over the past decade, empirical and theoretical studies on cumulative effects, especially on birds, have increased apace. We conducted a systematic review of the empirical literature on birds stored in the energy impact repository Tethys to determine how often different categories of evidence identified as important for understanding cumulative adverse effects have been measured. We used an evidence-mapping approach, aligning measurements recorded in Tethys with a published typology of evidence for cumulative adverse effects. Although many frameworks for understanding cumulative effects exist, rigorous quantification of measured outcomes is lacking. Our findings show that most empirical studies have identified which bird species are vulnerable to OSW effects (46%) or provided baseline data for species prior to OSW construction (40%), both of which are fundamental to understanding cumulative effects. In contrast, only 1% examined the nature of effect pathways (additive, synergistic, countervailing), and less than 3% sought to identify acceptable mortality thresholds for species populations affected by OSW. Additionally, there is a strong geographic bias toward the North Atlantic Ocean. Our analysis highlights critical gaps in the evidence required to understand the cumulative adverse effects of OSW on birds, guiding future research toward key areas where evidence is lacking.