Migration strategies of raptors – spatio-temporal adaptations and constraints in travelling and foraging

Sammanfattning: My dissertation is about bird migration and addresses questions about how raptors adapt to the fluctuating environments during the transition seasons between breeding and wintering. The migration is highly affected by local prey abundance, foraging strategies, weather conditions and landscape patterns along the migration routes as well as navigation and orientation mechanisms and cues. Interesting findings in my studies were how raptors can mix foraging and active migration during their travels. A strategy of fly-and-forage migration is favourable for birds that hunt on their wings, able to combine foraging with covering travel distance. Fly-and-forage migration is favourable for Ospreys in Europe because benefits (energy intake) more than outweigh costs (reduced flight time). Among different species and populations of migratory raptors, duration of migration increased approximately in proportion to the square root of total migration distance. This reflects a shifting balance in the selection for speed and duration depending on distance of migration. The general wind patterns along the migration routes with dominating winds from the east over the Saharan Desert and from the west over Europe, mediate migration in a clockwise loop, where Marsh Harriers could increase flight speed across the desert with tailwinds in autumn and by avoiding headwinds in spring. In addition to seas and deserts, we found the equatorial rain forest as a potential ecological barrier for migrating birds. A striking relationship of route convergence with the distribution of continuous rain forest suggests minimized crossings of this habitat by Hobbies. The migration journeys by raptors across the Sahara Desert showed that 37% of all crossings included events of aberrant behaviour indicating difficulties or hazards. Mortality associated with the Sahara passages contributed to about half of the total annual mortality among juveniles as well as adults, demonstrating that this passage has a profound influence on survival and fitness of the migrants. Comparisons of satellite tracking and ring recoveries of raptors revealed agreement in the geographical distribution but differences in timing of migration. Tracks showed a faster progress for long-distance migrants, which probably related to more detailed information from tracking in Africa, difficulties in judging time of death for recovered birds and an overrepresentation of recoveries in Europe. An important and challenging goal for future studies of raptor migration will be to track juveniles from their first journeys until they are migrating as experienced adults. This would give a better understanding of how migration strategies are developed and to what degree they are inherited or based on learning.

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