The high relevance of host–switching for the diversification of highly host–specific symbionts (i.e., those commonly inhabiting a single host species) demands a better understanding of host–switching dynamics at an ecological scale. Here, DNA metabarcoding was used to study feather mites on passerine birds in Spain, sequencing mtDNA (COI) for 25,540 individual mites (representing 64 species) from 1,130 birds (representing 71 species).

Host-shift speciation and cospeciation are the two major processes driving symbiont macroevolutionary diversification. Cospeciation is expected to be frequent in vertically transmitted and host-specific symbionts, and leads to congruent host-symbiont phylogenies. However, the cophylogenetic dynamics of many groups of highly specialized host-specific symbionts is largely unstudied. Thus, the relevance of cospeciation vs. host-shift speciation remains largely unknown. Here, this question is...