Background/Question/Methods Compared to simple gas diffusion models, biological systems add new because of the large spatial scales across which deterministic patterns emerge, and the fact that biological “particles,” i.e. organisms, have the ability to reproduce and propagate new species. Studies of body size have sought to understand how energetics both cause large-scale spatial patterns and also influence diversification. One natural and relatively unexploited experiment, analogous to diffusion of gasses between containers, is the spread of taxa across major landmasses. In the hopes of using insights from such an “experiment,” we ask (1) is the probability of a taxon being distributed across two continents determined in part by its body size and thus energetics? (2) After intercontinental dispersal or vicariance, is a taxon's subsequent diversification determined by its body size? And (3) is the evolution of a taxon's body size controlled more by phylogenetics or environment? Using data on birds of North and South America we simulated subsamples of body sizes and compared frequencies of size change and diversity for sister taxa on each continent. Results/Conclusions Through simulation we found that the body size distributions of genera with sister taxa on both continents are indistinguishable from a random sample of each continent's body size distribution. These results suggest that body size does not influence species diffusion and that perhaps body size is more a result of regional environments. However, we are not yet able to fully reveal whether continent of origin or phylogenetic constraints determine body size distributions in the absence of more comprehensive phylogenetic analyses. Sister taxa from temperate regions of both continents are significantly larger than endemic temperate taxa. If dispersal is assumed to be more limited in temperate zones simply on the basis of greater isolation-by-distance, then it appears that energetics may be conditionally important in determining diffusion and diversification of species: large body size may facilitate longer distance dispersal and vicariance. By comparing frequencies of size change and diversity in North and South America we find that the greater diversity of sister taxa in South America is not due to any difference in body size. Combined, our results imply that energetics play a conditional role in species diffusion across land masses, but that the greater significance may be found in how local to regional environment-energetics interactions “scale up” to processes of speciation which may be more directly related to large-scale biogeographic patterns.