Joanna Sudyka, Irene Di Lecce, Marta Szulkin

Animals breeding in anthropogenic shelters such as nest-boxes experience nesting environment in which they did not originally evolve. Over the past decades, they are additionally challenged by climate change – a major environmental force influencing their reproductive ecology. Despite the central importance of nesting microclimate for offspring development and fitness, very little is known about the thermal properties of human-provided nests compared to natural ones. While it has been demonstrated that artificial nests provide poorer thermal insulation in comparison to natural breeding sites, there is no evidence on how these shifts are shaped along the reproductive stages. In particular, comparisons focusing on the time when offspring are in the nest are lacking. Here, we compare the microclimatic conditions (temperature and absolute humidity) along the nesting cycle (from nest-site choice in early spring until post-fledging) in natural cavities and nest-boxes used by several species of hollow-nesting passerines in a temperate deciduous forest. We confirm that across all nesting stages, nest-boxes are thermally unstable when compared to natural cavities, with higher temperature maximums, larger amplitudes and worse insulation from maximum ambient temperatures relative to natural cavities. Importantly, in the presence of young, and after they start to thermoregulate on their own, nest-boxes are also more humid than natural cavities. Artificial nest microclimate is likely to amplify the adverse effects of projected temperature increases modelled under climate change scenarios, specifically by compromising thermoregulation and increasing water requirements of developing animals. In contrast, internal microclimatic shifts were mitigated in natural cavities 3.0 times more effectively than in nest-boxes when offspring were in the nest (in terms of mean daily differences from ambient temperature). We stress that conservation efforts should focus on the protection of habitats offering natural breeding-hollows to reduce climate change impacts on breeding animals.