Idiosyncrasy of feather mite intensity and prevalence across passerine bird species: a comparative study

Symbionts are key elements of ecosystems, by playing important roles in shaping the biology and ecology of their hosts. However, the factors determining symbiont loads across host species are still unclear. Nowadays, we know that the intensity and prevalence of feather mites, the most diverse group of avian ectosymbionts, differ strongly between species. To understand those differences, we studied 17 bird species traits and two feather mite community features (species richness and composition in bird species) potentially related to interspecific differences in feather mite intensity and prevalence across bird species. We analyzed a large dataset of feather mite occurrence across European passerines: for feather mite intensity a sample of 27 424 birds from 119 bird species, and 1 805 566 feather mites counted, and for prevalence a sample of 76 126 birds from 122 bird species. Feather mite intensity and prevalence covaried positively across bird species, reinforcing the previous observation that species differ in feather mite load. Comparative analyses of the association of bird traits and feather mite richness with feather mite intensity or prevalence showed a moderate explanatory power, high model selection uncertainty, inconsistent results for both prevalence and intensity data, and contrasting results with previous comparative studies on feather mite occurrence. Furthermore, even though closely related bird species had similar feather mite communities, there was a low evolutionary conservatism of both the prevalence and intensity of feather mites across bird species (i.e. low phylogenetic signal). Finally, feather mite community composition did not explain feather mite intensity or prevalence across bird species. Overall, our large number of host and mite traits failed to satisfactorily explain the high variability in feather mite occurrence across bird species. We discuss that this difficult-to-understand apparent idiosyncrasy of bird species in their feather mite intensity and prevalence may be the outcome of the interaction between host and symbiont traits.