Satellite Tracking Of Swedish Ospreys Pandion Haliaetus

Satellite tracking of Swedish Ospreys Pandion haliaetus: autumn migration routes and orientation
Mikael Hake, Nils Kjellen and Thomas Alerstam
Hake. M.. Kjellen. N. and Alerstam. T. 2001. Satellite tracking of Swedish Ospreys Pandion haliaetus: autumn migration routes and orientation. J. Avian Biol. 32: 47-56.
Autumn migration routes and orientation of Swedish Ospreys Pandion haliaetus werestudied by satellite tracking of 18 birds. Of these, 13 could be followed during the entire migration (6 females, 5 males and 2 juveniles). Most birds migrated across western and central Europe to winter in tropical West Africa. However, one juvenile flew to Cameroon and one female used a very easterly route and reached Mozambique. On average, the birds travelled a total distance of about 6700 km.with little variation except for the female wintering in Mozambique, who travelled more than 10000 km. Of 21 stopovers (of > I day), only five were made south of 45°N: three of these in Africa. Females departed before males and juveniles and llew to a stopover site they probably were familiar with. After 3 4 weeks there, they continued to their wintering grounds. Also males and juveniles usuallymade one or more stopovers. Adults seemed to travel to a known wintering site, where they remained stationary, whereas juveniles were more mobile after reaching tropical regions, probably looking for good wintering sites. Males generally left the breeding area in directions similar to the mean migratory direction, whereas a few females departed in diverging initial directions. Apart from thesediversions, adult Ospreys followed very straight migratory routes, with overall mean directions of 185-209° and with mean angular deviations of 6-33°. Some juveniles also departed in diverging directions. Moreover, young birds tended lo show a larger variability in orientation. Thus, the Ospreys kept a fairly straight direction and did not avoid geographical obstacles such as mountain ranges anddesert areas. However, they seemed reluctant to cross large water bodies. There was no correlation between angular deviation and length of the migrational segment, indicating that the principles of orientation by vector summation may not be valid for Osprey migration. Moreover, the geographic direction of migration did not vary in accordance with variations in the magnetic declination, suggestingthat the Ospreys did not orient along magnetic loxodromes.
Mikael Hake. Grimso Wildlife Research Station. Department oj Conservation Biology. SLU, SE-730 91 Riddarhyttan. Sweden. E-mail: niikael.hake@iuh.slii.se. Nils Kjellen and Thomas Alerstam. Department of Animal Ecoloev. Eeoloav BuUdine. SE-223 62 Lund. Sweden.
Cameroon (Osterlof 1977, Saurola 1995). However, there are also a number of recordsfrom East Africa itnd at least lour from South Africa, mainly of birds ringed in Finland, but also of Swedish birds (Saurola 1994. Stolt et al. 1998). Moreover, one bird ringed as a chick in northern Norway was found in India during the winter (NOF 1988). showing that the migration of the species is complex and not fully understood.
The Osprey Pandion haliaetus has a wide distribution in thenorthern hemisphere, but also breeds in the tropics. Most, however, breed in temperate areas, mainly between latitudes 40° and 70°N, and are longdistance migrants to tropical regions (Cramp and Simmons 1980. Poole 1980). Ringing recoveries show that most Ospreys breeding in northern Europe migrate across western and central Europe to winter in tropical West Africa, from Mauretania eastwards to
■JOURNAL OI- AVIAN BIOLOGY
Sweden holds a large proportion of the European breeding population, with an estimated 3200 pairs (SOF 1990). and this population is a potentially important source of birds for the re-establishment of populations in other parts of Europe, including Scotland (Dennis 1995) and Germany (Schmidt and Kapler 1994). Recently, there has been a tendency towards a population...