The tropical fowl mite, <i>Ornithonyssus bursa</i>, is a common avian parasite found on diverse bird species worldwide. In the Neotropical region, <i>O. bursa</i> is present in wild birds, but it may also infect poultry and bite humans. Little is known about the ecology and epidemiology of this parasite. We conducted a thorough longitudinal study in passerine assemblages from central Argentina, gathering data from six reproductive seasons, with the aim of identifying factors that have a role in driving the occurrence and distribution of <i>O. bursa</i> in its natural hosts. We focused on the brood and microhabitat levels, accounting for potential confounders of higher levels. The results hereby presented contribute to our knowledge on the eco-epidemiology of <i>O. bursa</i> in natural hosts of the Neotropical region. Among the many variables assessed, nest material and host species appeared to be the most important correlates of <i>O. bursa</i> prevalence. Nonetheless, supplementary analyses showed that host species is a stronger predictor than nest material. Moreover, mite burden (parasite intensity) was found to depend on host species, but not on nest material. The association with species depended on nestling age, suggesting that resistance builds up as the nestling develop, but at a different pace depending on the bird species. Brood size was inversely correlated with intensity of parasitism, suggesting a dilution of the parasite burden on each nestling.Facultad de Ciencias Naturales y MuseoCentro de Estudios Parasitológicos y de Vectores