Environmental Drivers on Blue Tit Nest Microbiome: An Experimental Study

Microclimate inside avian nesting cavities provides suitable growth conditions for microbial communities, which in turn may play a crucial role in influencing the well-being of the host. In this study, we investigated the microbiome of Blue Tit (Cyanistes caeruleus) nests subjected to experimental manipulations of temperature and humidity, aiming to evaluate the impact of these factors on fungal and bacterial communities. Additionally, we examined the associations between these microbial communities, parasitism and nesting birds' condition. Our results, based on metabarcoding analysis using 16S rRNA and the ITS2 region, indicated that while bacterial alpha diversity remained unaffected by the experimental manipulation, beta diversity differed significantly, particularly between nests with increased humidity and control nests. Similarly, elevated temperature and humidity increased fungal richness (alpha diversity) and altered fungal composition (beta diversity). We also observed that the abundances of bacterial and fungal phyla varied between treatments, with the differences being most pronounced in the case of fungi. We did not detect significant differences in potentially pathogenic bacteria between treatments. However, potentially pathogenic fungi, including dermatophytes, proliferated in humidified nests, potentially contributing to poorer nestling body condition compared to other nest treatment groups. The study also revealed significant correlations between microbial communities, ectoparasites and nestling body condition, indicating their potential interconnection. To our knowledge, this study represents the first experimental analysis of the microclimate effects on the nest microbiome. These findings highlight the complex interactions between nest microclimate, microbial diversity, ectoparasites and nestling development, offering new insights into the ecological effects of microclimatic conditions in avian nesting environments.