A View from Inside and Outside the Tree: Using Different Techniques to Assess the Use of Black Woodpecker Cavities in Forests

Large cavities are important for maintaining forest biodiversity. However, accurately assessing cavity use and identifying the communities of organisms associated with these structures remain challenging and are still insufficiently studied. Between 2022 and 2026, tree cavities excavated by the Black Woodpecker (Dryocopus martius) in northeastern Poland were monitored using two complementary techniques: camera traps installed near cavity entrances and an action camera used to inspect cavity interiors. The aim of this study was to assess the diversity of animal communities associated with Black Woodpecker cavities and to compare the effectiveness of the two monitoring methods. In total, 32 bird species, six mammal species, and 11 insect species were recorded as using Black Woodpecker cavities. Inspections of cavity interiors confirmed breeding by 19 bird species and 4 mammal species, and 4 insect species. Camera traps recorded 29 bird species, four mammal species, and one species of social insect at cavity entrances. Over half (51.7%) of the bird species recorded at cavity entrances were not detected inside the cavities, whereas 17.6% of bird species confirmed to breed in the cavities were not detected by camera traps. Camera traps also did not allow for the identification of bat species or the detection of insects other than hornets. The results demonstrate that the assemblages of animals associated with Black Woodpecker cavities differed depending on the monitoring method used. Traditional observation methods may underestimate the diversity of cavity-associated fauna, particularly species that remain hidden inside cavities or use them only during specific stages of their life cycle. The two modern techniques used in this study provided complementary information and should therefore be considered together when assessing the diversity and use of tree cavities by forest fauna.