Although the causes and consequences of social dominance have been examined extensively, avian studies have rarely focused onbetween-population differences in social dominance. On the island of Corsica, two resident blue tit Parus caeruleus populations25 km apart differ significantly in body size measures, timing and effort of reproduction, and song structure, and someof these population differences have a genetic basis. Because earlier avian studies have shown that social dominance is influencedby body size or mass, we predicted that individuals from these two blue tit populations would also differ in their ability todominate other individuals. Consistent with this prediction, we found that male blue tits of these two populations differ in socialdominance, and that heavier or larger individuals dominate lighter or smaller ones in aviary experiments. We propose thatsocial dominance may serve to maintain phenotypic population differentiation at a micro-geographic scale by acting as a barrierto dispersal.