Within large taxonomic assemblages, the number of species with adult body mass Mis characterized by a broad but asymmetric distribution, with the largest mass being orders of magnitude larger than the typical mass. This canonical shape canbe explained by cladogenetic diffusion that is bounded below by a hard limit on viable species mass and above by extinction risks that increase weakly with mass. Here we introduce and analytically solve a simplified cladogenetic diffusion model. When appropriately parameterized, the diffusion-reaction equation predicts mass distributions that are in good agreement with data on 4,002 terrestrial mammals from the late Quaternary and 8,617 extant bird species. Under this model, we show that a specific trade-off between the strength of within-lineage drift toward larger masses (Cope’s rule) and the increased risk of extinction from increased mass is necessary to produce realistic mass distributions for both taxa. We then make several predictions about the evolution of avian species masses.