In addition to the well-known limiting effect of cavity abundance on the density of hole-nesting passerines, other aspects of cavity availability may shape their communities as well. Notably, where there is a considerable aggregation of cavities, territory-holders may prevent the occupation of the nearest cavities by other birds, whereas a supply of diverse cavities may reduce interspecific competition. We used multivariate general linear models to explore whether, and how, variables describing the supply of small cavities are related to the density and diversity of hole-nesting passerine communities in 33 hemiboreal old forest stands. The total density of 12 species (1.3 ± 0.8 pairs/ha) increased with cavity density and diversity, but was not affected by cavity aggregation. As expected, cavity diversity also promoted bird diversity; indeed, the densities of different species were positively related to the densities of different cavity types. The results indicate that segregation in nest-cavity selection affects the co-occurrence of passerine species and, at the mean densities of small tree-cavities in the region (2.3/ha), cavity aggregation does not markedly reduce their availability. In conservation management, therefore, it is important to maintain a diverse supply of cavities in addition to their abundance, in order to sustain hole-nester communities.