Diversity patterns in eastern Nearctic and western Palaearctic temperate and boreal breeding assemblages of forest birds are compared. Several spatial scales of diversity were considered. First, the number of forest passerine species present in continental and regional (200 × 200 km2) pools were counted; secondly, average dissimilarities between breeding assemblages of passerine birds within a forest type (pine dominated forests) on a continent wide scale are calculated to assess the compositional variation among regional pools on each continent. Thirdly, the between-habitat diversity was assessed by comparing census results from two different forests types within regions. Finally, within-habitat diversity was measured by estimating the number of species present in standard samples from Nearctic and Palaearctic forests. Breeding assemblages of passerine birds of the eastern Nearctic region are more diverse than those of the western Palaearctic region on several spatial scales. First, there are more forest bird species in continental and in most regional species pools. Secondly, Neartic regional species pools are not only richer but also structurally more variable as revealed by a larger dissimilarity among Nearctic than Palaearctic pine forest bird assemblages. Thirdly, dissimilarity in bird community structure between two forest types is larger in the Nearctics, indicating a higher between-habitat diversity there. However, local breeding bird assemblages are about equally rich in species on both continents. The results match well with knowledge of the differences between the two continents in the history of avifaunas and vegetation formations. The results imply that conservation strategies should be uniquely tailored to local communities, taking into account the ecological characteristics of the species.
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