Microbiota and the Volatile Profile of Avian Nests Associate with Each Other and with the Intensity of Parasitism.

Bacteria have been suggested as being partially responsible for avian-nest odours and, thus, volatiles from their metabolism could influence the intensity of selection pressures due to parasites detecting olfactory cues of their hosts. Here, we tested this hypothesis by exploring intra- and interspecific variability in microbial environments, volatile profiles, and intensity of ectoparasitism by Carnus hemapterus in nests of ten avian species. 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 nest microbiota, as well as some particular bacteria and volatiles, was correlated to 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. Relative abundances of particular volatiles and bacteria predicted ectoparasitism and/or fledging success. Future research should prioritize experimental approaches directed to determine the role of bacteria and volatiles in the outcomes of host-ectoparasite interactions.