SPRING MIGRATION OF THE KNOT CALIDRIS-C-CANUTUS OVER SOUTHERN SCANDINAVIA, AS RECORDED BY RADAR

The bird migration pattern in June in southern Scandinavia was analysed on the basis of registrations from six years 1986-1991 by a long-range surveillance radar located in Scania, southern Sweden. The timing of migration, field observations and identification of some echoes suggested that mainly Knots Calidris c. canutus, on their way between the Wadden Sea and breeding grounds along the high arctic coasts of Siberia, were involved in the movements analysed. The daily timing was similar each year, with a peak passage across Scania in the night. The median time of passage (all years combined) was three hours after local sunset. The Knots have probably flown directly from well-known staging sites in the German Wadden Sea, and the median departure time from there was estimated to occur 1 1/2 h before sunset. The average track direction was 63-degrees +/- 14-degrees angular deviation. This is slightly, but significantly, to the south of the constant compass bearing (rhumbline = 57-degrees) between the German Wadden Sea and possible destination areas on the north coast of the Taymyr Peninsula in Siberia. Track directions varied significantly with wind, suggesting partial drift or pseudodrift. The groundspeed (25.6 m s-1) was on average 6.1 or 7.6 m s-1 (depending on the assumed flight altitude) faster than the estimated airspeed, showing that the majority of flocks had considerable tailwind assistance. Significantly more echoes were recorded in tailwinds than in winds from other directions relative to the average track direction. Evaluation of course changes of echoes tracked while crossing five degrees of longitude, over distances up to 400 km, showed that the birds did not follow any of the predicted paths according to known orientation mechanisms (sun-, star-, magnetic compasses). Rather, the overall migration route may be influenced by topography in a way that maximizes association with coastal areas.