Trematodes of Land Birds from the Republic of Mordovia with a Checklist of Avian Trematodes of the Middle Volga Region (European Russia)

We studied the trematode fauna in land birds from the Republic of Mordovia (European Russia) in 2018–2022. In total, we identified 16 digenean species in 45 species of birds from the orders Passeriformes, Piciformes, Caprimulgiformes and Falconiformes. The trematodes <i>Phaneropsolus micrococcus</i> and <i>Morishitum polonicum</i> were recorded for the first time in the birds’ parasite fauna of Russia. We obtained the first data on helminths in <i>Hippolais icterina</i> and <i>Ficedula albicollis</i> from Russia and in <i>Coccothraustes coccothraustes</i> from the Middle Volga region. New host records resulting from our study include <i>Brachylaima mesostoma</i> from <i>Coccothraustes coccothraustes</i>; <i>Urogonimus macrostomus</i> from <i>Sylvia atricapilla</i>, <i>Ficedula albicollis</i>, <i>Ficedula hypoleuca</i> and <i>Acrocephalus palustris</i>; <i>Plagiorchis maculosus</i> from <i>Ficedula albicollis</i> and <i>Hippolais icterina</i>; and <i>Lyperosomum alaudae</i> from <i>Ficedula hypoleuca</i>. The common parasite of rallid birds <i>Leucochloridium holostomum</i> is recorded for the first time from <i>Turdus merula</i> in Russia. Taking into account the newly obtained data, we carried out a review of trematodes in land birds of the Middle Volga region, of which the Republic of Mordovia is a part. Currently, the list of land bird digeneans in the Middle Volga region includes 56 species. Among all the studied land birds, members of the order Passeriformes have the richest trematode fauna (33 species). The diversity of trematodes found in passerines is due to the large number of both individuals and species studied and the variety of habitats and diet preferences of these land birds. Most of the identified trematode faunas (47 species) are obligate parasites of land birds. Nine species parasitize land birds accidentally and/or facultatively. In the Middle Volga area, the fauna of trematodes is the most diverse in land birds of the Nizhny Novgorod region, where 31 species are revealed. Fewer species of trematodes are identified in birds from the Bashkortostan (20), Mordovia (17) and Samara regions (15). For the birds of Chuvashia and Tatarstan, only eight and one species of trematodes are known, respectively. Six trematode species, found in land birds, have veterinary and medical significance as potential pathogens of dangerous helminthiases.