Nest quality limits the number of hole-nesting passerines in their natural cavity-rich habitat

We experimentally tested whether the lack of high-quality nest holes may limit the number of secondary cavity-nesters in cavity-rich habitats. We mapped and measured natural tree-holes in 10 mature aspen forests in Estonia. In five experimental plots, we provided nest-boxes on trees having suitable cavities for hole-nesting passerines. This improved the quality of available cavities but retained their total number (due to territoriality, sites in the same tree could not be occupied simultaneously) and location on the landscape. In the next breeding season, the density of hole-nesting passerines increased in experimental plots but remained at the same level in control plots. The increase was due to the species that used boxes: Parus major that preferred these over natural cavities, and Ficedula hypoleuca that used both nest types more opportunistically. We concluded that the quality of cavities can influence the numbers of hole-nesting birds in old cavity-rich forests, also; probably, adaptive habitat selection is the proximate mechanism in this case.