Cavities excavated by woodpeckers limit populations of other cavity-nesting birds

Research Highlight: Trzcinski, M., Cockle, K., Norris, A., Edworthy, A., Wiebe, K., & Martin, K. (2022). Woodpeckers maintain the diversity of cavity-nesting vertebrates in a temperate forest. Journal of Animal Ecology, https://doi.org/10.1111/1365-2656.13626. Whether populations of hole-nesting birds are limited by the availability of cavities is a long-standing, fundamental question in avian community ecology. The structure of cavity-nesting communities, known as nest webs, includes links between tree species that provide natural holes and bird species that nest in those holes (secondary cavity nesters, SCNs), tree species that provide substrates for cavity excavation and bird species such as woodpeckers that create cavities using those substrates (primary cavity nesters, PCNs), and between PCNs and SCNs. Trzcinski et al. (2022) focus on the latter links and provide the most compelling empirical evidence to date that cavities created by PCNs specifically, as opposed to natural holes or holes generally, limit populations of SCNs. Using data from a long-term study, the authors employ three analyses to separate effects of availability of cavities from environmental factors such as food, habitat features and host tree abundance, while controlling for annual variation and autocorrelation within sites, to isolate the relationship between excavated cavities and SCN numbers. They show that nest density of SCNs is positively related to PCN nest density the previous year, an indicator of availability of excavated cavities, and that the effect of PCNs is stronger when other variables are accounted for. North American coniferous forests such as that studied by Trzcinski et al. (2022) are exceptional in that excavated cavities comprise the vast majority of nesting holes. Whether their findings apply to other systems in which PCNs are a major source of cavities, or to particular PCN-SCN relationships in systems in which excavated cavities account for a much lower proportion of nesting holes remains to be investigated.