Wind farms along the migration route of birds act as unnatural barriers, and avoiding them during flight may require the expenditure of extra energy. Information regarding cumulative effects of barriers on migrating birds is generally lacking, mainly because of the complexities of monitoring the number of encounters of migratory birds with wind farms and their flight path for avoiding these barriers. It would be desirable to develop a general method for monitoring the rate at which migratory birds encounter wind farms. In this study, we attempted to assess the potential cumulative barrier effects on 17 eastern buzzards (Buteo japonicus) and eight Oriental honey-buzzards (Pernis ptilorhynchus) using global positioning system (GPS) tracking data. We obtained the location data of wind turbines in Japan and migration paths of the birds using GPS loggers and assumed four scenarios that birds could use to avoid the wind turbines along their routes. Although the number of studied individuals was limited and the impact of the cumulative effects are inconclusive at the present stage, the estimated additional distance, time, and energetic cost during one migration were no more than 31.97 km, 75.74 min and 132 kJ, respectively, which were relatively small. Additionally, we showed the possibility that GPS tracking could provide information on migration episodes of birds associated with wind farms and could be a promising method for addressing the cumulative barrier effects. We expect that a higher sampling frequency of locations would enable precise measurement of avoidance flights and energetic costs.
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