Birds’ nests may be refuges for various species of fungi including that which are potentially phytopathogenic and zoopathogenic. Among the 2449 isolates of fungi obtained from nests of Marsh harriers 96.8% belonged to filamentous fungi. In total, 37 genera were identified from 63 fungi species. Within the mycobiotas of the examined nests populations of fungi which are potentially pathogenic for humans, homoiothermous animals and plants dominated. Among 63 species, 46 (72%) were potentially pathogenic fungi of which 18 species were potentially phytopathogenic and 32 species were pathogenic for homoiothermous animals. Inter alia species of fungi were found in the Marsh harriers nests: Aspergillus fumigatus, Aspergillus flavus, Scopulariopsis brevicaulis, Chrysosporium keratinophilum and Fusarium poae, Fusarium sporotrichioides. In terms of numbers, dominant in Marsh harrier nests were fungi pathogenic to birds, other homoiothermous animals and humans. On that basis it was concluded that Marsh harrier nests are both a source of fungal infections for that species and one of the links in the epidemiological cycle of opportunistic fungi for humans.