ABSTRACT. – We studied the structure of breeding and wintering bird assemblages in a Mediterranean wetland of Central Italy with the aim of evaluating seasonal structural changes and the role of habitat heterogeneity on the avian community. The wintering assemblage showed higher values of species richness, diversity and evenness. The seasonal differences were represented through a diversity/dominance approach (species rank/relative occurrence diagram or Whittaker plots), and the curve obtained for winter showed a significantly shallower slope if compared with that for the breeding season (Kolmogorov-Smirnov test). K-dominance plots also revealed a different shape in the accumulation curve of the relative occurrence values between wintering and breeding season. Whittaker’s β-diversity was higher in winter. In Mediterranean wetlands, intrinsic constraints (e.g. phenology and ecology of the species) seem to be crucial in determining the community structure and the shape of diversity/dominance curves. In winter, many species are vagrant and sparsely distributed, and belong to species-rich taxonomic groups (e.g., waders, ducks), while in the breeding season most of species are common and more uniformly spread. Extrinsic constraints also affect bird assemblages in winter: the presence of water induces an increase of habitat heterogeneity, expressed by the β-diversity index. These habitat changes induce an increase of occurrence values, species richness, diversity and evenness. The shape of the seasonal curves in Whittaker plots followed the MacArthur broken-stick model.