Satellite tracking of Swedish Ospreys Pandion haliaetus: Autumn migration routes and orientation

Autumn migration routes and orientation of Swedish Ospreys Pandion haliaetu s were studied by satellite tracking of 18 birds. Of these. 13 could be fol lowed during the entire migration (6 females, 5 males and 2 juveniles). Mos t birds migrated across western and central Europe to winter in tropical We st Africa. However, one juvenile flew to Cameroon and one Female used a ver y easterly route and reached Mozambique. On average, the birds travelled a total distance of about 6700 km, with little variation except for the femal e wintering in Mozambique, who travelled more than 10000 km. Of 21 stopover s (of > 1 day), only five were made south of 45 degreesN; three of these in Africa. Females departed before males and juveniles and flew to a stopover site they probably were familiar with. After 3-4 weeks there, they continu ed to their wintering grounds. Also males and juveniles usually made one or more stopovers. Adults seemed to travel to a known wintering site. where t hey remained stationary, whereas juveniles were more mobile after reaching tropical regions, probably looking for good wintering sites. Males generall y left the breeding area in directions similar to the mean migratory direct ion, whereas a fen females departed in diverging initial directions. Apart from these diversions, adult Ospreys followed very straight migratory route s. with overall mean directions of 185-209 degrees and with mean angular de viations of 6-33 degrees. Some juveniles also departed in diverging directi ons. Moreover, young birds tended to show a larger variability in orientati on. Thus, the Ospreys kept a fairly straight direction and did not avoid ge ographical obstacles such as mountain ranges and desert areas. However. the y seemed reluctant to cross large water bodies. There was no correlation be tween angular deviation and length of the migrational segment, indicating t hat the principles of orientation by vector summation may not be valid for Osprey migration. Moreover, the geographic direction of migration did not v ary in accordance with variations in the magnetic declination, suggesting t hat the Ospreys did not orient along magnetic loxodromes.