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).

The consequences of symbiont transmission strategies are better understood than their adaptive causes. Feather mites are permanent ectosymbionts of birds assumed to transmit mainly vertically and massively from parents to offspring. This may seem maladaptive because of the low survival expectancies of chicks compared to bird parents. Why, then, do feather mites have this mass transmission to nestlings? Here, the transmission of Proctophyllodes doleophyes is studied in two European populations...