Physiological constraints related to atmospheric temperature pose a limit to body size in endothermic animals. This relationship has been summarized by two classical principles of biogeography: Bergmann’s and Allen’s rules. Body size may also constrain other phenotypic traits important in ecology, evolution and behaviour, and such effects have seldom been investigated at a continental scale. Through a multilevel-modelling approach, we demonstrate that continent-wide morphology of related African barbets follows predictions of both ecogeographic rules, and that body size mirrors variation in song pitch, an acoustic trait important in species recognition and sexual selection. Bergmann’s rule effects on song dwarf those of acoustic adaptation at a continental scale. Our findings suggest that macroecological patterns of body size can influence phenotypic traits important in ecology and evolution, and provide a baseline for further studies on the effects of environmental change on bird song.