Long-term trends in bird populations: a review of patterns and potential drivers in North America and Europe

Data from breeding bird monitoring schemes provided material for numerous studies to relate the trends of particular species to their ecological and life history traits. This review contains a comprehensive comparison of results of these studies, describes the patterns in bird population trends in North America and Europe in last forty years and discusses potential drivers. I omitted other sources of bird population trend estimates to reduce methodological bias and because bird monitoring studies are rarely represented in other parts of the world. The most intensively studied driver is habitat alteration on breeding grounds represented by agricultural intensification in Western Europe and North American grasslands, forest expansion and land abandonment in Northern, Southern and Eastern Europe and parts of North America, and more localized urbanization and forest habitat fragmentation. Evidence for climate change impacts is robust and includes almost all European regions covered by the reviewed studies but the causal mechanisms of its influence on bird abundance are not sufficiently known. Population limitation by environmental changes in wintering quarters is well supported by the studies covering 1970s and 1980s, whereas the effects of migration strategy became less important from 1990s onwards. Support for other potential drivers, such as direct disturbance by humans, is rather limited and further studies are needed to confirm their effects.