Avian taxi drivers? A commentary on ‘Diversity of fungi attached to birds corresponds to the habitat ecologies of their avian dispersal vectors’

Birds are increasingly recognized not only as seed dispersers, but also as important agents of fungal dispersal. Although their role in seed dispersal, particularly through frugivory, has been studied extensively owing to its significance for long-distance dispersal and forest regeneration (Spiegel and Nathan, 2007), their role in transporting fungi has received comparatively little attention. Early work by Heald and Studhalter (1913) documented birds dispersing plant pathogenic fungi such as Cryphonectria parasitica, the causative agent of chestnut blight. However, research on bird–fungal interactions has lagged behind that of bird–plant relationships. Recent findings suggest that avian-mediated fungal dispersal is far more widespread and ecologically important than previously recognized (for a comprehensive review, see Elliott et al., 2019). Despite this growing recognition, the diversity of fungi dispersed by birds and the mechanisms underlying these interactions remain poorly understood.

In this issue of Annals of Botany, Johansson et al. (2025) provide new insight on how the habitat ecology of birds influences the fungal communities they disperse. The birds examined in this study are two migratory passerines with contrasting habitat ecologies: European Robin (Erithacus rubecula; Fig. 1A) and Goldcrest (Regulus regulus). European Robins forage mostly on the ground of semi-urban forests and parklands dominated by deciduous trees, whereas Goldcrests forage mostly in the canopy of conifer-dominated forests. Using an environmental DNA-based survey, Johansson et al. (2025) show that although both bird species carry a large number of ubiquitous environmental fungi, they also carry substrate-specific fungi. These substrate-specific fungi include a wide diversity of taxa and ecologies, including plant pathogens, saprotrophs and mutualists, and are linked to the different habitats that the birds occupy (deciduous vs. conifer forest). Thus, each bird species harbours distinct fungal assemblages. Dispersal has traditionally been considered an unimportant process in structuring microbial communities; however, there is ample evidence that dispersal limitation influences microbial diversity (Peay et al., 2010). In fact, most airborne spores move only a few metres from their source, and although long-distance aerial dispersal of spores is possible, it is uncommon. For this reason, animal-mediated movement of fungal propagules can be particularly relevant for plant-associated fungi, which often exhibit host specificity and might benefit from more targeted, directed movement (Vašutová et al., 2019). The distribution of plant-associated fungi is directly influenced by the distribution of their host. Therefore, animals associated with a particular habitat type are more likely to move fungal propagules to a suitable habitat where the plant host is present. The findings of Johansson et al. (2025) underscore how habitat use by birds not only shapes the composition of fungal communities they transport but also highlights their potential role in the long-distance dispersal of ecologically significant plant-associated fungi.

Fig. 1.

(A) European robin (Erithacus rubecula). Robins spend a considerable amount of time on the ground hunting for invertebrates but are also considered opportunistically frugivorous [photograph: Ryan Schain, Macaulay Library at the Cornell Lab of Ornithology (ML34750391)]. (B) Galls produced by Exobasidium vaccinii on Vaccinium vitis-idaea (Ericaceae), which was detected by Johansson et al. (2025) (photograph: Jouko Rikkinen). (C) Leaf spots caused by Exobasidium sydowianum on Arctostaphylus uva-ursi (Ericaceae) (photograph: Jouko Rikkinen). (D) Coloured gall produced by Exobasidium ferrugineae on Lyonia fruticosa (Ericaceae) (photograph: C. Benton Willis).

Several studies have suggested that migratory birds might be relevant for long-distance dispersal of plant pathogenic fungi. Interestingly, European Robins examined by Johansson et al. (2025) carried Exobasidium species in their feet. Exobasidium is a genus of biotrophic plant pathogens that cause galls or leaf spots primarily in the family Ericaceae (Fig. 1B–D). Many species of Exobasidium form colourful spore-producing galls that might attract dispersers (Fig. 1D). However, the dispersal mechanism of Exobasidium species remains unclear. Other important plant pathogens, including Heterobasidion spp. and Lophodermium spp., were also detected on both European Robins and Goldcrests. Heterobasidion species cause root rot and pose a serious threat to conifers such as Pinus and Picea, whereas Lophodermium species are foliar pathogens responsible for needle cast in conifers. Interestingly, Heterobasidion species have also been found on Red-cockaded Woodpeckers (Dryobates borealis) in the southeastern USA. Although Heterobasidion and Lophodermium species are thought to rely primarily on short-distance aerial dispersal, these avian-mediated movements might facilitate long-distance dispersal of these pathogens. It is also worth noting that birds might carry fungi as spores but also as vegetative tissue. For example, many birds in the tropics disperse root-like vegetative fungal tissue (i.e. rhizomorphs) when building nests, including some pathogenic species (Koch et al., 2020).

Moreover, Johansson et al. (2025) show that, besides plant pathogenic fungi, European Robins and Goldcrests carry a diverse assembly of saprotrophic and lichen-forming fungi. Saprotrophic fungi carried by these birds include a diversity of species ranging from litter saprotrophs, such as Mycena, to wood-decaying fungi, such as Peniophora. Although these fungal species are likely to be aerially dispersed at short distance, the presence of these fungi on bird feet and feathers suggest that they might, at least, facilitate dispersal. Other studies have looked at the impact that birds have on saprotrophic fungi. Red-crockaded woodpeckers are known to shape the communities of saprotrophic fungi in their nesting cavities (Jusino et al., 2016). The high prevalence of lichen-forming fungi, such as Scoliciosporum, suggests that birds could also facilitate long-distance dispersal of lichens.

Although saprotrophic, plant pathogenic and lichen-forming fungi were commonly found on the feet and feathers of both European Robins and Goldcrests, mycorrhizal fungi were detected far less frequently. However, it would be interesting to see whether an increased sample size or a different sampling scheme could change that trend. Correia et al. (2019) showed that European Robins and other European birds, such as the Sardinian warbler (Curruca melanocephala), are able to co-disperse seeds of elmleaf blackberry (Rubus ulmifolius) and their associated arbuscular mycorrhizal fungi. Likewise, other passerines, such as Chucao Tapaculo (Scelorchilus rubecula) and black-throated huet-huet (Pteroptochos tarnii) in southern South America, disperse ectomycorrhizal fungi by eating their fruiting bodies (Caiafa et al., 2021).

Johansson et al. (2025) provide new insight on the complexities of bird-dispersed fungi, even with a relatively small sample size. Differentiating ubiquitous environmental fungi from the fungi that are actively being dispersed by birds is a necessary step to understand the ecological relevance of avian-mediated fungal dispersal systems. This study adds to the growing body of work on avian–fungal interactions, and the focus on functional groups of fungi is notable. Nonetheless, there are still some missing pieces and directions for future studies. First, although environmental DNA sequencing greatly facilitates this type of research, it would be useful also to incorporate a microscopical examination of the samples. Caiafa et al. (2021) adopted such an approach and not only captured spore morphology but also demonstrated spore viability in avian faecal samples. The direct observation of fungal spores and hyphae in future studies will provide more definitive proof that birds are moving fungal propagules and not only DNA. Second, experimental approaches can help to elucidate the impacts of these bird-mediated fungal movements on fungal communities on plants and soil. Greenhouse experiments, similar to the ones described by Correia et al. (2019), could provide evidence that birds carry viable propagules of plant-associated fungi. This is not an easy endeavour, and patterns arising from this type of research are not always easy to disentangle, but this is the only way to understand the complexities of avian-mediated fungal dispersal systems. I am excited to see this field moving forwards and cannot wait to see where it goes in the coming years.