Flying with the wind - Spring migration of Arctic-breeding waders and geese over South Sweden

Spring migration of arctic breeding birds, mainly waders and Dark-bellied Brent Geese Branta bernicla, was followed by tracking radar in Lund, South Sweden, during 1998-2001 in order to evaluate the capacity of these birds to exploit favourable winds. A total of 888 radar tracks were collected. Here I report on measured flight speeds, flight altitudes and winds and compare these with results from similar studies of arctic waders from around the world. Ground speeds were generally high, showing a strong support by following winds. The support by tailwinds was on average 8.3 ra s(-1). In total, 95% of the tracked flocks were flying with supporting winds. Flight altitudes ranged between 100 m and 3745 m above ground with a grand mean of 1750 m. Birds flying at higher altitude experienced a stronger wind support than birds An MITI- flying at lower altitude. Winds experienced by the birds differed from overall wind conditions during the seasons in question, the former showing a higher concentration in the most favourable sector (winds between south and west). The results indicate that these birds are selective in their choice of migration days with respect to wind, i.e. that they actively choose to migrate during days, and probably also at altitudes, giving them a good wind support. A relatively large proportion of the waders were also found to use strong tailwinds, exceeding the birds' own airspeeds, which support the suggestion of a risk-prone time-selected migration strategy in this group of birds.