Numerous forest organisms critically depend on availability of tree cavities. Some birds and rodents fill their cavities with bulky nests, which – if not removed – could accumulate and render cavities unusable, as recorded in nest-box studies. Data from earlier studies indicate
that old nests can disappear from tree cavities so fast that practically no remnants are detectable the following spring. Rapid decomposition of nests in situ, augmented by physical removal of nest material by the cavity-users have been proposed as possible causative mechanisms. We
tested these hypotheses in cavities used by tits (Parus major L., Poecile palustris L.) in the Białowieża National Park (E Poland). To mimic typical components of their nests – moss and mammalian hair – we inserted into cavities litter-bags filled with
either cellulose or fleece of sheep. After 9,5 months of exposure a third of bags were missing from cavities, and 92–100% of cellulose and 84–98% of hair disappeared from the remaining ones. These results confirm that the rapid decomposition of the tit nests in cavities reinforced
by material removed by vertebrates would suffice to “clean” the cavities between the consecutive seasons. To conclude, accumulation of litter in natural nests holes does not seem to affect nest hole availability.