On the role of migration for the distribution of arctic birds - a circumpolar perspective

Sammanfattning: Flexibility as well as constraints to the evolution of migration routes has the possibility of affecting large-scale patterns of geographical ranges of animals both by facilitating, but sometimes restricting, accessibility to and colonization of new regions. This dissertation concerns the broad-scale effects migration has on the distributional patterns of birds in the Arctic. I studied the circumpolar patterns of associations between spatial variation in species richness of shorebirds and environmental and ecological factors, including migratory flyway diversity (chapter I), occupancy and spread of shorebird populations between different tundra regions and major winter regions as associated with ecological barriers, migratory distances and relative location of major stopover habitats (chapters II & III). Furthermore, I have looked at associations between migratory distance status and longitudinal extent of all arctic bird species’ breeding ranges (chapter IV) and the structuring of areas with high species spatial turnover in the arctic bird communities of groups restricted to different winter habitats (chapter V). The evolution of inter-continental bird migration and its importance for shaping broad patterns of community composition is discussed (chapter V). During a ship-based expedition to the Beringia region, the pattern of large-scale intercontinental bird migration of arctic breeding birds between the Old and New World continents was explored using ship-based radar (chapter VI). Finally, (in chapter VII), a method for assessing whether adults and juveniles of flocking shorebirds segregate or aggregate during fall migration is presented and theoretical associations between migration route flexibility and social versus genetical inheritance depending on breeding latitude and migration distance are discussed. The main results of the different studies suggest that migration has a profound importance for the distributional structure of the arctic avifauna in several ways. Migration, or rather the dependence on crucial non-breeding habitats, affects arctic distributions by limiting the accessibility of different arctic regions in different ways for different groups (for shorebirds chapters I-III and for marine, terrestrial and coastal groups of birds chapters IV-V). Important factors, in common for the groups considered are the extent and location of ecological barriers to migration, though specific to each group, and the relative location of crucial non-breeding grounds. There are, however, also some patterns of distribution that seem to require historical explanations reflecting limitations to colonization that probably stem from inertia in the evolutionary flexibility of migration routes.