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.