Migratory birds choose a certain route to their destination from potential routes under different conditions. Wind conditions are one of the most important factors affecting the choice via costs and risks of migration. This study investigates if the detour by the Grey-faced Buzzard (Butastur indicus) breeding in Japan is adaptive in terms of migration costs and risks. To this end, we simulated flights of individuals along four possible routes using wind data, and calculated costs, defined as time to complete migration, and risks as probabilities not to reach breeding sites before the breeding season. Despite the large difference in distance, the difference in the costs between the detour and the shortest route, which had the lowest costs, was less than 40 hours. Risks did not vary greatly among the routes. Moreover, the detour has another advantage, the larger proportion of distance over land to the entire distance. Based on these facts, the detour seems to be the most adaptive among the four routes. Our approach explicitly accounts for the effect of wind conditions on bird flights. It would allow the more accurate evaluation of costs and risks than one based on distance only, eventually serving to a better understanding of the evolution of migration routes.