From unwanted squatters to good tenants: Ectosymbionts and their relationships with body condition of Atlantic Forest Passeriformes

Many organisms live in or on birds, with arthropods being the best studied ones. Ectosymbiotic arthropods may parasitise birds by feeding upon blood or feathers, thereby causing injury or competing for essential resources. Nevertheless, ectosymbionts sometimes cause no adverse effect to birds because they consume only excess preening oil, debris and microorganisms on feathers. The influence of these ectosymbionts on host body condition is still poorly understood, especially for species living in one of the most endangered biomes, the Atlantic Forest in South America. We tested for the relationships between ectosymbionts and host’s morphological traits: flight muscle condition, mass index and feather growth rate. We inspected the body surface of 282 individual birds of 45 understory‐dwelling species looking for ectosymbionts and estimated vane‐dwelling feather mite load in their remiges. We calculated ectosymbiont prevalence and mean infestation intensity in 193 individual birds from 11 species. In seven species (n = 106 individual birds), we tested for the relationship between ectosymbionts and morphological traits using generalised linear mixed models. We found a negative relationship between the presence of ectoparasites and the host pectoral muscle condition, supporting the parasitic nature of tissue‐feeding symbionts. Nevertheless, we found no relationship between vane‐dwelling feather mite loads and hosts’ morphological traits, which supports the hypothesis of likely commensalism between these organisms.