Numerous bird species depend on the availability of tree cavities, and most non-excavators fill their cavities with considerable amounts of nest material. If not removed, this material can accumulate and render cavities unusable, as recorded in some nest-box studies. Data from earlier studies of tree cavities, however, showed that nest material can decrease mostly due to in situ decomposition, but the relative difference between nest decomposition in tree holes and nest-boxes is still unknown. We undertook parallel studies of decay in tree holes and nest-boxes used by European Starlings Sturnus vulgaris in oak-hornbeam stands (SW Poland). We inserted into its tree holes and nest-boxes litter-bags filled with cellulose and wool. After 7.5 months of exposure we detected much greater decomposition in tree holes than in nest-boxes. In tree holes a median 75% of cellulose and 26% of wool disappeared, whilst in nest-boxes a median of only 2% of cellulose and 14% of wool. These results are the first to document the relative difference between natural and artificial breeding cavities in the extent of nest decomposition. We also discuss the effect of nest material accumulation in tree holes and nest-boxes on the different nesting conditions available for hole-nesting birds. Taken together with: microclimate, nest safety, competition with social insects and presence of ectoparasites, the physical accumulation of nest material appear to be distinctive feature that differentiates the natural and artificial sites of tree-hole-nesting birds.