Microclimate in tree cavities and nest-boxes: Implications for hole-nesting birds

The provision of nest-boxes is widely used as a conservation intervention to increase the availability of cavities for hole-nesting birds, particularly in managed forests, but it is uncertain whether nest-boxes are an appropriate substitute for tree cavities. Tree cavities and nest-boxes may differ in many aspects, including microclimate, but there are few data with which to examine this. We measured the air temperature and relative humidity in vacant tree cavities previously used by breeding marsh tits Poecile palustris (a non-excavating forest passerine) and in nest-boxes provided for this species that had similar dimensions to natural nest sites, and we compared values from both with ambient conditions. We examined how tree cavity characteristics influenced microclimate and if similar conditions were replicated in nest-boxes. Tree cavities, particularly those in thicker parts of trees, were more efficient thermal insulators, with temperature extremes dampened to a greater extent relative to ambient values. In contrast, the nest-boxes provided poor insulation with negligible buffering against ambient temperatures. Mean daily relative humidity was high (on average c. 90%) in tree cavities, which all had walls of living wood, and this averaged 24% higher than in nest-boxes at comparable ambient conditions (mean humidity 76-78%). These results support previous studies that incorporated various types of tree cavities and nest-boxes, indicating that the environment within nest-boxes differs significantly from that of tree cavities. We conclude that providing nest-boxes may affect microclimatic conditions available for cavity-users, which may have ecological implications for nesting birds.