Cavities used for avian reproduction in spring often host other organisms for roosting during winter, which should affect their microclimatic conditions and, then, the patterns of emergence of nest-dwelling ectoparasites of birds and successful parasitism. To prove these cascade effects, we experimentally blocked the entrance of half of nest boxes previously used for reproduction by hoopoes (Upupa epops), recorded temperature and humidity during the fall-winter period, and evaluated abundance, emergence of ectoparasitic flies (Carnus hemapterus), and intensity of parasitism in hoopoe nestlings growing in open and blocked nest boxes the next spring. As expected, experimental treatment as well as the occupancy of open nest boxes by rodents (i.e., mainly dormice [Eliomys quercinus]) affected temperature and humidity, but did not predict the onset or duration of Carnus emergence. Moreover, flies were more abundant in open nest boxes and, among them, those occupied by dormice showed the lowest abundance. Finally, hoopoe nestlings developing in nest boxes blocked during winter experienced lower intensity of ectoparasitism than those in open nest boxes. These hitherto undescribed cryptic effects of overwinter occupants of nest cavities on subsequent emergence and viability of nest-dwelling ectoparasites would profoundly impact on the interaction with their avian hosts.