Diversity patterns of breeding bird assemblages (exclusive of raptors and nocturnal species) of western North American oak and Australian eucalypt woodlands are derived from data recorded at 113 census sites distributed over four regions, two on each continent. Regional species richness varies by a factor of 2 among regions. The contribution to regional species totals by various diversity components is examined: α-diversity (species richness within sites), β-diversity (species turnover between sites related to differences in vegetation structure), and γ-diversity (turnover related to distance between sites, independent of habitat change). Mean α-diversity is relatively constant among regions (mean 25.5 to 29.7 species). Variation in α-diversity within regions is related to variation in vegetation structure, and bird-density variation is best predicted by a measure of vegetation density. The relationships between bird diversity and density and vegetation structure are similar in the four regions. With the influence of vegetation structure removed, there is no (Australia) or at best a modest (North America) latitudinal gradient in α-diversity. Within regions, regression analysis shows that species turnover is significantly related to both vegetation structural differences (β-diversity) and distance between sites (γ-diversity), with the latter accounting for a larger proportion of, and correlating strongly with, regional species totals. Identification of factors promoting species turnover between sites, beyond distance and vegetation effects, remains a major challenge to ecologists.