Bacteria metabolize organic compounds of nest materials and debris from reproductive activity that accumulate in avian nests during the breeding season, and, thus, have been suggested as being partially responsible for nest environment odours. Since ectoparasites mainly use olfaction to detect their hosts, bacteria located in avian nests (Nidobiome) and volatiles from their metabolism could influence the probability of parasites detecting such nests and, thus, the intensity of selection pressures due to parasites. Here, we tested this hypothesis by exploring intra- and interspecific variability in microbial environments and volatile profiles of nests in ten different avian species at the beginning and at the end of the nestling period, as well as the intensity of ectoparasitism by Carnus hemapterus flies suffered by nestlings. As expected, we found that (i) alpha and beta diversity of microbial and volatile profiles associated to each other. Moreover, (ii) alpha diversity of bacteria and volatiles of nest environment, as well as some particular bacteria and volatiles, associated with intensity of parasitism at early and late stage of the nestling period. Finally, (iii) alpha diversity of the microbiota of the nest, as well as some particular bacteria and volatiles, was correlated with fledging success. When considering them together, the results support the expected links between microbial environment and nest odours in different bird species, and between microbial environment and both ectoparasitism intensity and fledging success. Future research should prioritize experimental approaches directed to determine the role of bacteria and volatiles in the outcomes of host-ectoparasite interactions.