We tested a geometric constraint model, the mid domain effect (MDE), as an explanation for spatial gradient in species richness of Palearctic songbirds. A data set was created of digitized equal area distribution maps of 3036 phylogenetic species of Palearctic songbirds, and the emergent biogeographic pattern were analyzed over a one degree longitude equal area map of the Palearctic region., with a grid cell area of 4062 km. comparing the observed richness pattern among 2401 phylogenetic breeding taxa of songbirds in the Palearctic domain revealed that a fully stochastic bio-dimensional model has little empirical support for overall species richness of Palearctic songbirds. Furthermore, we evaluated two applied approaches to test the frequency distribution of geographic range sizes (RSFD) of individuals species within the Palearctic domain. Endemic Birds areas results show that these areas are more species richness than other areas, even when small-range species are excluded from overall measures of species richness. We confirmed that MDE models should no longer be used for assessing the role of MDE for species richness.

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