| Summary: | The world is heating up. The climate is changing, with increasing temperature changes towards the Arctic. Northern ecosystems of tundra and taiga are subject to changes, even in the most remote areas void of human presence. One of the most profound characteristics of these northern ecosystems are the cyclic changes in population size of mammals, birds and insects, small microtine rodents being the central species in the dynamics. Several studies have demonstrated that the cyclicity of lemmings and voles have changed during the recent decades, with consequences for many other species of the food webs. Changes in the cyclicity of lemmings and voles are especially expected to influence their predators. All arctic and boreal owl species hunt microtine rodents, and species like the Snowy Owl and the Great Grey Owl are totally dependent on such prey animals to breed. The Snowy Owl is listed as a vulnerable species worldwide, while the Great Grey Owl is considered to have a stable world population. The population of Snowy Owls breeding in Fennoscandia has declined while the Great Grey Owl recently has expanded its breeding distribution. Small mammal hunters like the Snowy Owl and the Great Grey Owl are directly influenced by changes in the cyclicity of microtines. The difference in population development of these two vole hunters in Fennoscandia enhance the importance of monitoring both species under a regime of expected future changes of ecosystem cyclisity. Knowledge of population size, reproduction and survival, and the age structure of populations are paramount information in such monitoring because reproduction and mortality varies with age. Moult patterns are essential for aging many birds. In species where juvenile and adult feathers look different, such differences can be used for aging a bird when it is captured for banding. In this thesis I present a method for aging Great Grey Owls and Snowy Owls based on the moult patterns in their wings. I have demonstrated the difference between the first juvenile wing ...
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