The air speed of migrating birds and its relationship to the wind

Summary This paper deals with the flight speeds of migrating birds observed by radar over a Swiss Alpine pass. The distributions of air speeds for different classes of birds and the influence of the wind on air speeds were investigated. Our findings differ greatly from observations of bird migration over the North Atlantic Ocean and the North American continent. Our data reveal: (1) that the air speeds of the bulk of migrating birds were in the range of 8–18 m/s and that the amount of slow flyers (air speed below 5 m/s) was less than 5%; (2) that there was an obvious influence of the wind on air speeds and, moreover, the data showed a distinctly variable compensatory behavior among different bird classes; (3) that the wind component in the direction of the bird's heading was a better predictor for air speed than the wind direction. Although we do not speculate about possibilities and mechanisms of wind detection, a simple argument shows how birds could estimate wind directions if they do use the moving surface as a reference system.