5. CONCLUSIONS
Our study demonstrates that the observed difference in the bacterial load of the nest box is probably related to habitat-related differences between the two study sites. The most important factors are probably associated with the richness of plant species and the physical properties of the soil, with additional factors resulting from the specificity of the two study sites. However, taking into account the complex and yet relatively weakly understood relations between microorganisms, their hosts, and the environment, probably other factors play their role and we are still far from understanding this issue. Furthermore, we should treat our results with care since we do not have data on the richness of bacterial species at both study sites. However, our study suggests that the bacterial load may be a potentially important factor for the life-history traits of secondary cavity nesting birds, since it may vary considerably between habitats. Therefore, since studies on cavity nesting birds with the use of artificial wooden nest boxes are widely conducted across a geographical range and a variety of habitats, and given that bacterial load may affect host fitness components, we consider our study perspective as a promising field for future research.