Advances in technology and engineering are enhancing the contribution that wind power makes to renewable energy generation. Wind farms, both operational and in planning, can be expected to impact negatively on wildlife populations, particularly birds. We propose a novel approach to assess the impacts through the energetic costs of avoidance behaviour for a long-distance, migratory seaduck. Flight trajectories were recorded using surveillance radar at a Danish offshore wind farm with emphasis placed on the 200 000+ migrating common eiders that pass through the area annually. Minimum distance to wind farm and curvature of trajectories were compared pre- and post-construction. Additional costs of the avoidance response were estimated using an avian energetic model. The curvature of eider trajectories was greatest post-construction and within 500 m of the wind farm, with a median curvature significantly greater than pre-construction, suggesting that the birds adjusted their flight paths in the presence of the wind farm. Additional distance travelled as a consequence of the wind farm's presence was ca. 500 m and trivial compared with the total costs of a migration episode of 1400 km. However, construction of further wind farms along the migration route could have cumulative effects on the population, especially when considered in combination with other human actions.