We’ve just launched our next experiment on coccidiosis looking for the possible transmission of isosporan parasites between members of closely related species, house (Passer domesticus) and tree sparrow (P. montanus). Provided that (1) the two species often form mixed flocks both in rural and urban areas and (2) coccidians are transmitted via faecal–oral route, it seems reasonable to expect cross-infestation to occur. Beyond testing this, we examine what physiological effects do have, if any, the coccidians of house sparrows on tree sparrows and inversely.