Experimental evidence of a consistent commensal relationship between avian hosts and feather mites of differing host specificities

The majority of research on birds and their symbionts has focused on putatively parasitic relationships, but this presumption often lacks experimental evidence and overlooks the potential for context-dependent patterns. Nearly all studies regarding the impact of feather mites on their avian hosts have been correlative and spatially or taxonomically limited, resulting in conflicting and narrow classifications of the nature of the symbiosis (i.e., parasitic, mutualistic, or commensal). Furthermore, gaps in understanding mite host specificity, and thus an inability to account for this ecologically and evolutionarily important trait, may also contribute to these varying conclusions. Here, we examined the impacts of this trait by experimentally removing feather mites from two species of wild wood-warblers that harbor mites with different host specificities (one host specific, one found on ≥17 host species). Birds were recaptured the following year to assess potential for context-dependent impacts of mites on host body condition and apparent annual survival. Our treatment (mite removal) did not impact either metric and thus supports the neutral effect (commensalism) of feather mites on both Setophaga cerulea (Cerulean Warblers; host of the generalist mite, Amerodectes ischyros) and Protonotaria citrea (Prothonotary Warblers; host of the specialist mite, A. protonotaria). We found that mite abundances on S. cerulea, but not P. citrea, rebounded on hosts after one year despite experimental manipulation, so it is unclear how long our treatment lasted. This population rebound also indicates the need to better understand basic life history of feather mites (e.g., reproductive or transmission rate). This study provides some of the only experimental data on this perplexing symbiosis as well as a preliminary assessment of the importance of host specificity on the functional nature of this symbiotic relationship.