Bird nests support a diverse, dynamic and distinctive macroarthropod community on the forest floor

•1. Nests built by vertebrates constitute diverse microhabitats that are inhabited by many taxa other than their owners. Hence, vertebrate nests can be important hubs of local biodiversity. •2. Arthropods are the largest group of additional nest users, forming complex networks of interspecific relationships; hence, the distribution of nest‐building animals and the availability of their nests may have a significant impact on local arthropod communities. •3. We investigated factors that may explain differences in macroarthropod communities in ephemeral nests of Wood Warblers, Phylloscopus sibilatrix (Bechstein, 1793), a declining ground‐nesting songbird, and compared them to leaf litter nearby. •4. In 2019–2020, we collected 168 warbler nests and 45 leaf litter samples in the primeval stands of the Białowieża Forest (Poland). From nests and litter samples, we extracted, respectively, 11759 and 349 macroarthropod specimens, which represented 183 and 70 species. •5. The composition of nest‐dwelling macroarthropods varied within and between years, and between the warbler's nesting stages, and with time since nest abandonment by the birds. Habitat type, nest size and material composition poorly explained differences in the nests' macroarthropod communities. Compared to leaf litter in the surrounding environment, warbler nests hosted distinctive, more abundant and slightly more diverse macroarthropod communities. •6. Our findings are likely to be relevant to a wide range of bird and mammal nests. Thus, the widespread decline of nest‐building vertebrates might reduce the availability of nest resources, potentially posing a previously overlooked threat to invertebrate conservation.