Flight altitudes of coastal birds in relation to wind direction and speed

During systematic sea watches carried out between 1 September and 15 November 1999 on the East Friesian island of Wangerooge, observations were recorded of the flight altitude of coastal birds in relation to wind direction and speed. In Redthroated Diver Gavia stellata, Common Eider Somateria mollissima and Common Scoter Melanitta nigra the proportion of birds flying into the wind low over the water (0-1.5m) increased with wind speed. On the other hand, in the same species, the number of low-flying birds decreased in inverse proportion to the speed of a tail wind and the ratio of birds flying at greater altitudes increased (1.5-12m and 12-25m respectively). Irrespective of wind speed, the proportion of individual birds flying low into the wind was highest in Red-throated Diver, Shelduck Tadorna tadorna, Common Eider and Common Scoter. This pattern is repeated at a higher level in Sandwich Tern Sterna sandvicensis and Common/Arctic Terns S. hirundo/paradisaea. In contrast, in tail winds, the greatest proportion of birds of these species invariably flew at the highest levels. Comparisons of flight altitudes reveal that these species fly noticeably higher in tail winds. This behaviour can be explained in terms of economy of effort on migration. The present study also reveals that diurnal movement of the observed species takes place mainly at a low flight altitude (up to 25m, occasionally up to 50m, rarely higher) above sea level. This demonstrates potentially adverse effects on birds from construction of proposed offshore wind farms. The data indicate that, to be of any value in the assessment of the potential disturbance of the wind farms to North Sea migrants, flight altitude records must be viewed against the background of the meteorological situation as a whole.