RESUMO
Faunistic information shows that Denmark, Norway, Sweden and Finland were colonized by an average of 2.8 species (winners) and lost 0.6 species (losers) per decade and country in 1850-1970. The smallest passerines had best changes of colonization, but body size and colonization success were not related in nonpasserines; extinctions were fairly common among the largest species. Turnover was highest in a faunal group derived from steppe regions and in widely but often patchily distributed species. Population trends were generally similar over a larger region. Species turnover has changed the total number of breeding pairs only a little, but the colonization rate has been highest after 1950, when quantitative population changes have also been extensive.Extinction was most often caused by persecution and/or habitat changes by man (e.g. destruction of old forests). As large nonpasserines are often hunted or persecuted, they were frequent among the losers, many of which have specialized habitat requirements and usually migrate to the tropics.Over 50 of the 88 winners have been favoured by man's direct measures, such as introductions (3 spp.), relaxed persecution, or improved food conditions. Habitat changes (eutrophication, reduced grazing, and conifer plantations) stand out as major factors and may subtly interact with population parameters. Perturbation of tropical habitats in this century may have benefitted many Palearctic winter visitors: the frequency of tropical migrants among the winners has increased from 10% before 1900 to 38% after 1900. Population changes in central parts of the range probably led to several colonizations and extinctions at the periphery.In theoretical terms, r-selected species (small body size, large clutch size) were often winners, while K-species comprised the bulk of the losers, but this is no more than a general trend. Judged by their faunal origin, species adapted to effective dispersal were often winners, but also seemed vulnerable to environmental deterioration. Because many winners have invaded habitats that have been available for a long time, North European bird communities are probably often unsaturated.
