Nesting cavities constitute micro-environments very likely to be colonized by ectoparasites which feed on blood of the incubating female and the nestlings. Given the negative impact of ectoparasites on nestlings there will be selection on hosts to minimize ectoparasite loads through behavioural defenses. We have addressed the implications of ectoparasitism in three sympatric avian cavity-nesters, namely Pied Flycatchers Ficedula hypoleuca, Blue Tits Cyanistes caeruleus and Nuthatches Sitta europaea, to explore if differences in prevalence and abundance of generalist ectoparasites (blowflies, fleas and mites) can be related to interspecific differences in their nest size, nest composition and cavity microclimate. Furthermore, we have aimed at detecting if interspecific variation in the incidence and intensity of anti-parasite behaviours is a consequence of the abundance of ectoparasites. Differences in nest composition among host species appear not to be the main factor explaining ectoparasite loads, while nest size, breeding phenology, brood size and nest-cavity micro-climate may affect them in different ways for each host-parasite association. Behavioural defenses against parasites are exhibited by all host species but are more intense in the host species with the highest infestation levels (Blue Tits). This study shows different sources of variation in associations between three sympatric avian cavity-nesters and their generalist ectoparasites.