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The nests of secondary cavity-nesters located in tree cavities may form specific microhabitats of conservation importance due to their limited accessibility and availability. Species-specific nesting materials in nests of different secondary cavity-nesters may furthermore provide very different microhabitats for arthropods. The potential differences in arthropod communities inhabiting nests of different bird species in excavated cavities or nest boxes have, however, rarely been studied despite their relevance for conservation. Here we investigated the diversity and composition of arthropod communities in these different cavity types and bird species' nests in managed boreal forests. We identified morphologically and by DNA-metabarcoding arthropods in nest materials that were collected in and compared between (i) woodpecker-size cavities from seven different combinations of cavity type (nest box or excavated cavity), tree species (aspen or pine) and accumulation history of nest materials (single-season cleaned or uncleaned nest boxes that accumulated nests of passerines or an owl species); and (ii) nests of two different passerine species in small nest boxes. We identified 64 arthropod taxa in ten orders, from which Diptera, Coleoptera, Siphonaptera, and Lepidoptera were the most abundant. Shannon diversity index was similar among the cavity-nest-type combinations, but taxa richness was the highest in the owl nests. The arthropod communities (especially Histeridae beetles) deviated most from the other types of nests in owl and aspen cavity nests with more advanced decomposition of nest material (soil or wet environment related taxa). The differences in arthropod communities between the different nest types point out the importance of the ecological chain "tree cavities—bird nests—arthropod communities". [ABSTRACT FROM AUTHOR]

Cavity nesting birds depend on the availability of tree cavities for breeding, but the structure of the surrounding environment may also impact cavity occupancy. Here we investigated the effects of forest structure on occupancy rates of excavated cavities and similar-size nest boxes by cavity nesters in managed southern boreal forests in Finland. We recorded the occupancy over five breeding seasons (2017–2021) and derived forest structure parameters from high-resolution thematic raster maps. We found a high occupancy rate in nest boxes (81%), but lower rates in natural cavities excavated in forest interior, forest edges and retention trees (42-46%). The analyzes focusing on Paridae only revealed that the occupancy rates in nest boxes were decreasing with increasing amounts of Norway spruce and deciduous tree foliage biomass, and tree height, but increasing with the proportion of Scots pine. In forest interior cavities, we found a negative effect of stand age but a positive effect of the proportion of spruce. We conclude that the benefits of nest boxes can be maximized by considering specific forest parameters when deploying them. In excavated cavities the occupancy rates are likely more dependent on the varying internal quality than on the forest structure. [ABSTRACT FROM AUTHOR]

• We compared habitat and distances among the cavities used for hoarding and for nesting. • Habitat composition affects the different cavity uses by the Eurasian pygmy owl. • Sex differences and main food availability are overlooked factors for tree cavity occupancy. • A pygmy owl needs a minimum of five cavities, in suitable nesting and hoarding habitats. • Forest management may have serious consequences on the pygmy owl populations in the boreal forest. Tree cavities are an essential requirement for reproduction and survival of secondary cavity nesters. Little is known, however, about how cavity distribution in space and time may limit their use during and outside of the breeding season. Using long-term (17 yr) data on the use of nest-boxes resembling tree cavities by individually marked Eurasian pygmy owls Glaucidium passerinum , we compared the habitat and the distances among the nest-boxes used during two different seasons. We found that pygmy owls are more demanding in their requirements for breeding than for food-hoarding habitat, as the habitat surrounding the nesting sites had a higher proportion and biomass of coniferous forests, especially spruce, and nesting sites were farther from houses than food-hoarding sites. Pygmy owls mainly used different nest-boxes for nesting and food-hoarding and they also used different nest-boxes in different years. The distance between the nest site and food-hoarding sites was longer when the main food availability (voles) was low than when it was high. The distance was also longer in females than in males, probably indicating that females need to move further to find suitable hunting areas. In conclusion, these results highlight the importance of spruce forests as a nesting habitat of pygmy owls, and the need of several tree cavities within their home range. Forest management should secure a sufficient supply of these resources at the scale of pygmy owl home ranges. [ABSTRACT FROM AUTHOR]

• We estimated the availability of tree cavities for pygmy owls in the boreal forest. • Number of natural cavities per se does not limit their breeding or food hoarding. • The owls are able to reuse cavities filled by other species by cleaning them. • The owls showed a preference for a thick front wall and a deep inner chamber. • Population limitation is not via hole availability but other habitat quality aspects. Natural cavities are a critical resource for non-excavating hole-nesting birds, many of which are declining in northern Europe. In the heavily managed boreal forests in Finland, cavities may be a scarce resource, but their availability to different hole-nesters has been examined in only a few studies. We studied the abundance of suitable cavities in two managed forest landscapes 300 km apart for a forest-dwelling predator, the Eurasian Pygmy Owl Glaucidium passerinum , which requires tree cavities for both nesting and food hoarding. We further examined the preferences of the owls for different cavity characteristics in two experiments and several years of monitoring data with nest boxes resembling natural cavities. We found that natural cavities were scarce in the landscape (6.5/km2). However, natural cavity abundance per se does not seem to limit the breeding density of this predator, as suggested by a low occupancy rate in natural cavities and nest boxes. Owls cleaned the cavities that were filled with nest material of other birds, which provides a greater number of possible cavities to choose from. Cavities whose characteristics prevent the nest from being reached by predators were clearly preferred, as the owls select cavities with more than 5 cm width of the front wall and 15–30 cm of entrance-bottom distance within the cavity. The entrance hole orientation or the height of the cavity from the ground did not affect cavity occupancy by pygmy owls for nesting. In addition, none of the characteristics of the cavities that we surveyed affected their use for food hoarding. While the number of cavities per se may not limit the nesting of the owls, they may be limited by suitable habitat with abundant food supply around available cavities, or by lack of cavities in suitable habitats. They also need more cavities in the winter than in the summer, as each individual needs one to six cavities for hoarding food to survive over winter. Effective conservation strategies for this forest specialist should therefore include the maintenance of high-quality habitat that contains suitable cavities, supporting the need for protection of mature and old-growth forests as a whole, and not just of cavity bearing trees alone. [ABSTRACT FROM AUTHOR]