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...