Staying sheltered: geographical variation in nestbox use by a nocturnal raptor outside the breeding season

Abstract

<p>Tree cavities and nestboxes are crucial resources for cavity-nesting birds. While cavities are essential for reproduction, their importance outside the breeding season remains poorly understood, with most existing evidence focusing on nestbox use by small birds to reduce heat loss during winter. Whether non-breeding cavity use varies geographically along latitudinal gradients is also largely unknown. Here, we conducted a three-year study on tawny owl <em>Strix aluco</em> to investigate nestbox use during autumn and winter using trail cameras in four populations located at different latitudes (Finland, Sweden, Slovenia, Italy). We predicted non-breeding nestbox use to be a daytime behaviour, and to be more common under colder climatic conditions, in more deciduous habitats and in areas with higher predator (both avian and mammalian) and mobber abundance. We indeed find that tawny owls primarily use nestboxes during the day and in winter, but use nestboxes rarely outside the breeding season, except in Italy. PCA-based analyses of population differences in climate, forest habitat and bird community provides some support for the notion that tawny owl nestbox use outside the breeding season is mainly associated with deciduous forest, possibly because these provide less cover in winter. In addition, tawny owl nestbox use is mainly observed in active territories and winter nestbox use is associated with the probability of starting to breed in the subsequent breeding season. Our findings imply nestboxes, and presumably also natural cavities, have multiple ecological functions for tawny owls outside their breeding season. A better understanding of year-round nestbox use is important for developing effective wildlife conservation strategies in forest ecosystems.<br></p>