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Within regions, differences in the number of species among clades must be explained by clade age, net diversification rate, or immigration. We examine these alternatives by assessing historical causes of the low diversity of a bird parvorder in the Himalayas (the core Corvoidea, 57 species present), relative to its more species rich sister clade (the Passerida, ∼400 species present), which together comprise the oscine passerines within this region. The core Corvoidea contain ecologically diverse species spanning a large range of body sizes and elevations. Despite this diversity, on the basis of ecological, morphological, and phylogenetic information, we infer that the best explanation for the low number of species within the Himalayan core Corvoidea is one in which ecology limits diversification and/or immigration. Within the core Corvoidea, body size is correlated with elevation: large species are found at high elevations, and small species at lower elevations. This contrasts with the presence of many small-bodied species spanning all elevations in the Passerida and many large bodied species at low elevations in the other orders of birds (the nonpasserines). Cladogenetic events leading to ecological differences between species in body size and shape mostly occurred millions of years ago, and the rate of evolutionary change has declined toward the present. Elevational distributions have been evolutionarily more labile, but are also associated with ancient cladogenetic events. We suggest the core Corvoidea occupy a restricted volume of ecological space in competition with other bird species, and this has limited in situ diversification and/or immigration. [ABSTRACT FROM AUTHOR]