Birds are hosts for a wide variety of ectosymbionts (including mites and lice) and it is commonly presumed that most of these invertebrates have co-evolved with their avian hosts. However, very little is known about the details of these intimate relationships (for mites in particular), including the identity of the ectosymbiont species, the nature of the relationships (mutualistic, commensalistic, or parasitic?), and the level of concordance between the phylogenies of the ectosymbionts and their avian hosts (e.g., perfect co-evolution or frequent host-switching?). With many collaborators, we are investigating the effect of mites on avian fitness and construct the co-evolutionary history between mites and one family of birds, the Parulidae (New World Warblers). Although the co-evolutionary default for such host-specific symbionts may be perfect concordance between the lineages, we are testing several hypotheses that could explain patterns of host-switching that we have uncovered.