On the importance of farmland as stopover habitat for migrating birds

Abstract: Agricultural intensification has led to dramatic population declines among many farmland bird species. Negative effects of intensive farming in terms of decreased local habitat quality or structural simplification of farmland landscapes have reduced both reproductive outcome and winter survival of many species. Studies investigating agricultural effects on migrating birds using farmland as stopover habitat are scarce and done only on a limited number of species, despite the fact that stopover conditions not only affect the rest of the migratory journey but due to carry-over effects often also the future fitness of the birds. In this thesis, I used a quantitative and a qualitative approach to assess the importance of farmland as stopover habitat for avian migrants. The quantitative approach showed that an impressive number of birds use farmland as stopover habitat, and that some species occur in large proportions of their flyway populations. I also found that organic farming can enhance both abundance and species richness of migrants. However, its effects are both trait- and landscape-dependent indicating differential benefits for different species. For the qualitative approach, I studied the stopover ecology of Golden Plovers, a tundra breeding wader that during migration occurs in large flocks in intensively farmed areas. Using observations, ringing data and radio-telemetry, I found that individual plovers stay for up to several months to finish flight feather moult and put on fuel before further migration. The birds prefer large open fields with short vegetation both for diurnal roosting and for nocturnal feeding. Grasslands, in previous studies often associated with feeding plovers, are avoided. Although staging Golden Plovers respond to spatial and temporal variation in food availability, these results indicate that the species seem to manage well in intensively farmed arable landscapes. In conclusion, this thesis shows that farmland is an important stopover habitat for a significant number of avian migrants. Thus, changes in farming conditions that may affect habitat quality, e.g. agricultural intensification, may have profound impacts on whole bird populations. Although nothing can be said about the qualitative importance of farmland for other species than the Golden Plover, my studies show that organic farming and landscape complexity can influence numbers and species richness of migrants. However, the effects of these two factors are trait-dependent and may interact with each other, which should be considered when developing and evaluating agri-environment schemes. Overall, a varied agricultural landscape at all spatial scales is likely to have the most beneficial effect on migrating birds and biodiversity in general.