Living birds represent the only extant sauropsid group in which pulmonary air sacs pneumatize the postcranial skeleton. Notable in this regard is an extraordinary degree of variability, ranging from species that are completely apneumatic to those characterized by air within the entire postcranial skeleton. Although numerous factors (e.g., body size) have been linked with "relative" pneumaticity, comparative studies examining this system remain sparse. This project sought to (1) characterize whole-body patterns of skeletal pneumaticity in distantly related neognath birds and (2) evaluate putative relationships among relative pneumaticity, body size and locomotor specializations. Pneumaticity profiles were established for 52 species representing 10 higher-level groups. Although comparisons reveal relatively conserved patterns within most lower-level clades, apparent size- and locomotor-thresholds do impart predictable deviations from the clade norm. For example, the largest flying birds (vultures, pelicans) exhibit hyperpneumaticity (i.e., pneumaticity of distal limb segments) relative to smaller members of their respective clades. In contrast, skeletal pneumaticity has been independently lost in multiple lineages of diving specialists (e.g., penguins, auks). The application of pneumaticity profiling to extinct archosaurs reveals similar trends in body size evolution, particularly when examining patterns of pneumaticity in a size-diverse assemblage of pterosaurs (flying "reptiles"). As a fundamental organizing system, skeletal pneumaticity may play a role in relaxing constraints on body size evolution by allowing volumetric increases without concomitant increases in body mass. Not only might this be critical for taxa (birds, pterosaurs) exploiting the energetically costly aerial environment, but could be beneficial for any large-bodied terrestrial vertebrates such as the dinosaurs. (c) 2009 Wiley-Liss, Inc.