Parasitology · 9 authors, 8 centres
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This study examined fecal samples from 755 wild passerine birds across 13 sites in Britain and Ireland, finding an overall intestinal parasite prevalence of 20.0%. Parasite prevalence and abundance varied significantly by host family, season, and host age, with juveniles showing higher infection rates than adults. The presence of bird feeders was associated with higher infection intensity, suggesting supplementary feeding may increase parasite transmission risk.
**Background:** Avian endoparasites play important roles in biodiversity, behaviour, ecology, host evolution and species conservation. Understanding host–parasite associations is essential for predicting patterns of parasite emergence, transmission and pathogenicity. Despite the importance of wild birds as potential reservoirs for generalist parasites, there is a scarcity of epidemiological studies on intestinal parasites infecting wild passerine populations, particularly in Britain and Ireland. Prior research has often focused on livestock, pets, and humans, with few studies examining both helminths and protozoa in wild birds simultaneously. The effects of ecological variables such as anthropogenic food availability, habitat diversity, and diet on parasite prevalence and abundance remain poorly understood.
**Methods:** Fecal samples were collected from 755 wild passerine birds caught during regular ringing operations over 15 months (March 2020 to June 2021) at 13 sites across England (n=9), Wales (n=3), and the Republic of Ireland (n=1). Sites consisted of garden (n=6), woodland (n=2), or farmland (n=5) habitats. Birds were caught using whoosh or mist nets, fitted with British Trust for Ornithology metal rings, and aged/sexed where possible. Feces were collected using sterile holding bags and refrigerated within 8 hours. Laboratory analysis used conventional fecal flotation with sodium nitrate (specific gravity 1.2) and the McMaster egg counting technique. Parasites were identified morphologically to genus level. Three phylogenetically controlled generalised linear mixed-effects models (PGLMMs) using MCMCglmm were constructed to analyse parasite prevalence, abundance, and infection intensity, controlling for host phylogeny and time until analysis. Fixed effects included bird family, host age, season, geographic region, feeder presence, diet diversity, and habitat diversity. Rarefaction analyses and co-occurrence analyses were also performed.
**Key Results:** Fourteen parasite genera were identified: gregarines (Monocystis), coccidians (Eimeria, Isospora), nematodes (Capillaria, Porrocaecum, Syngamus), cestodes (Anonchotaenia, Dilepis, Passerilepis, Variolepis), and trematodes (Brachydistomum, Collyriclum, Echinostoma, Leucochloridium). Overall intestinal parasite prevalence was 20.0% (151/755). The most detected genera were Syngamus spp. (33.1% of infected; 50/151), Isospora spp. (32.4%; 49/151), and Capillaria spp. (27.2%; 41/151). Prevalence differed significantly between avian families (range: 0–70%; mean ± s.e.: 23.2 ± 5.6%), with corvids, starlings, and dunnocks showing highest prevalence. Seasonal variation was significant: winter had the highest prevalence (26.1 ± 3.4%) and spring the lowest (16.5 ± 2.3%). Juveniles had higher prevalence (22.4%; 89/397) than adults (17.9%; 51/285). Diet diversity was strongly associated with prevalence (pMCMC = 0.023), with species consuming 6 diet types having the highest prevalence (75.0%; 6/8). Mean parasite abundance was 1704.0 ± 504.8 EPG/OPG, with Isospora having the highest abundance (1485.2 ± 500.51 OPG). Abundance varied by family, season, habitat diversity (pMCMC = 0.024), and age (pMCMC = 0.032). Mean infection intensity was 8407.0 ± 2420.0 EPG/OPG, with Isospora showing the highest maximum intensity (7327.5 ± 2417.7). Infection intensity was influenced by geographical region, habitat diversity (pMCMC = 0.048), and feeder presence (pMCMC = 0.044); samples from sites with feeders had greater infection intensity (9847.1 ± 2990.6 EPG/OPG) than those without (3894.1 ± 2370.9 EPG/OPG). Most infected individuals had single infections (87.4%; 132/151), with 12.6% (19/151) having coinfections. Co-occurrence analyses found positive associations between Monocystis and Syngamus, and negative associations among 6 parasite pairs, with Isospora negatively associated with 4 other parasites.
**Clinical Implications:** This study provides the first comprehensive epidemiological survey of both helminthic and protozoan intestinal parasites in wild passerine birds across Britain and Ireland. The overall 20% prevalence is lower than reported in some other regions but comparable to studies of resident wild birds. The finding that juveniles have higher prevalence and abundance than adults suggests age-related immunity development. The association between bird feeder presence and higher infection intensity highlights potential risks of supplementary feeding for parasite transmission, particularly relevant for conservation and garden bird management. Seasonal variation, with highest prevalence in winter, may reflect reduced immune investment during resource-limited periods or increased sociality. The negative associations between Isospora and multiple helminth genera suggest possible immunological competition or cross-protective immunity. The study underscores the importance of continued surveillance, as anthropogenic activities and climate change may alter host–parasite dynamics, with implications for the One Health approach given the global distribution of passerines and potential for zoonotic disease transmission.