Breeding Ospreys were studied in southern Sweden and 13 birds were tracked by satellite telemetry on autumn migration to the African wintering grounds. This was supplemented with studies of migrating birds at Falsterbo and radar trackings From southern Sweden. Females generally left the nest site 2-3 weeks ahead of males and juveniles. Among males, failed bleeders migrated significantly earlier than successful breeders. At Falsterbo, Ospreys passed in the order adult females (median 22 Aug), adult males (26 Aug) and juveniles (30 Aug). Birds tracked by radar achieved cross-country speeds of 18-47 km/h. Most of our birds wintered in an area from The Gambia to the Ivory Coast, with one juvenile in Cameroon and one Female in Mozambique. Ospreys spent on average 45 days travelling an average distance of 6742 km with no significant differences between sex and age categories. Between 0 and 44 days were used for stopovers en route. Females generally made more stopovers at northerly latitudes than males. Average speed on migration was 174 km/d, which is similar to speeds reported for other large raptors followed by satellite. Speed on travelling days was on average 257 km/d with males generally moving fastest. There was a clear tendency for lower speeds and more stopovers in Europe than during the crossing of the Sahara. Migratory activity generally rook place between 8 a. m. and 5 p. m. local time and we have no indications of birds flying at night. With 9 hours travelling rime the expected cross-country speed, derived from the theor of thermal soaring flight and assuming thermal climb rates of 1-2 m/s, varies from 251 to 360 km/d, which is similar to the observed mean spaed on travelling days. Even so, one male travelled 746 km/d between Sweden and Spain. Some Ospreys need a much larger fraction of travelling days than expected from theory, suggesting that they deposit fuel on the breeding grounds before departure. This is supported by a correlation between the observed fraction of days spent travelling and departure date. In late departing Ospreys, especially males and juveniles, a major part of the energy for migration is probably deposited on the breeding grounds.
- Biologiska vetenskaper