Tree cavities are an essential resource for cavity-dwelling mammals, birds, invertebrates and fungi, and so are important for maintaining forest biodiversity. In North American forests, woodpeckers (Picidae) play a keystone role in cavity creation by excavating holes. However, in European forests many hole-nesting songbirds rely on non-excavated cavities that are formed by fungal decay and compartmentalization after tree damage. Several factors are recognised in initiating non-excavated cavities that are used by hole-nesting birds, including loss of a tree branch or stem breakage, but this topic is poorly studied. Here, we propose that bark stripping by large herbivores (e.g. Red Deer Cervus elaphus and European Bison Bison bonasus) could be another important, and previously overlooked, mechanism for initiating tree cavities that are used by hole-nesting birds. We suggest that, after the initial damage from herbivore bark-stripping, fungal decay can create specific elongated, slit-like cavities, which are particularly important as nest sites for some common forest songbirds. We outline this idea using original observations and evidence from the literature, primarily from the primeval forest in Poland's Białowieża National Park. We also use studies from elsewhere in Europe to show a generally low usage of slit cavities by birds where large herbivores are scarce or absent. We suggest that restoring such animals in European forests could help to restore the abundance and diversity of the tree cavity resource for hole-dwelling species. We encourage future research to investigate this proposal of large herbivores being important agents of tree cavity formation that could enhance biodiversity.