Host space, not energy or symbiont size, constrains feather mite abundance across passerine bird species

Abstract

1. Comprehending symbiont abundance among host species is a major ecologicalendeavour, and the metabolic theory of ecology has been proposed to under-stand what constrains symbiont populations.2. We parameterized metabolic theory equations to investigate how bird species'body size and the body size of their feather mites relate to mite abundance ac-cording to four potential energy (uropygial gland size) and space constraints (wingarea, total length of barbs and number of feather barbs). Predictions were com-pared with the empirical scaling of feather mite abundance across 106 passerinebird species (26,604 individual birds sampled), using phylogenetic modelling andquantile regression.3. Feather mite abundance was strongly constrained by host space (number offeather barbs) but not by energy. Moreover, feather mite species' body size wasunrelated to the body size of their host species.4. We discuss the implications of our results for our understanding of the bird–feather mite system and for symbiont abundance in general.