Many animals seasonally travel between their breeding and wintering grounds. With their advanced mobility, birds often migrate over thousands of kilometres. Recently, satellite-tracking studies have revealed peculiar migration routes for some avian species at a global scale. However, the adaptability of such migration routes has not been clearly demonstrated. Using satellite-tracking data for 33 individuals, we show that the Japanese population of Oriental honey-buzzards (Pernis ptilorhynchus) directly crosses the 650-km-wide East China Sea during their autumn migration, although they fly a longer route around the sea rather than directly crossing it during their spring migration. By applying aerodynamic theory, we show that the buzzards could cross the sea by soaring and gliding flight. Moreover, using a high-resolution meteorologicalprediction analysis, we demonstrate that the migratory trajectory of the birds strongly depends on the wind direction at their estimated locations. In the area, northeastern tailwinds blow stably only during autumn. Thermals were abundant ca. 500-1,000 m over the East China Sea in autumn, but that was not the case in spring. We suggest that the autumnmigration route across the East China Sea is likely to have evolved in response to the specific weather conditions over the sea. Animations showing movements of Oriental honeybuzzards and temporal change in weather conditions are available at:

This paper introduces user-friendly atmospheric data, so-called objective analysis data, for ornithological studies. The data have been interpolated onto grid points distributed at a regular interval in space and time, and are suitable for analyzing using computer data analysis software. Data assimilation techniques, which are basically the application of optimization and control theories, are utilized for producing objective analysis data in order to reduce errors as much as possible and obtain the most reliable dataset. Some examples of objective analysis data are shown and their features are described. Some cautionary notes are also given in order to avoid misinterpretation of the data.