Omnivorous waterbirds play an important role in aquatic ecosystems as dispersal vectors via direct ingestion, transportation, and egestion of plant and invertebrate propagules (i.e. endozoochory). Predatory birds also have the potential to disperse plants and invertebrates that were first carried internally or externally by their prey animals. However, the potential contribution of predatory waterbird species to propagule dispersal in aquatic ecosystems remains understudied. We chose the grey heron Ardea cinerea (Ardeidae) to study the potential of predatory waterbirds to disperse propagules within and among aquatic ecosystems. We hypothesised that: (1) herons disperse a wide variety of plant and invertebrate propagules, from different habitats, with different morphologies (i.e. dispersal syndromes), and including both native and alien species; (2) propagules are ingested with prey species that are primary dispersal vectors (i.e., herons are secondary dispersers); (3) heron pellets show a similar abundance and richness of propagules across their widespread range. We collected 73 regurgitated heron pellets containing undigestible remains from 12 locations across the U.K. and The Netherlands, and examined the taxonomic diversity of plant seeds, invertebrates and prey remains. Pellets were dominated by mammal hairs (99% by volume), and bones confirmed the ingestion of small mammals (prevalence of 38%, e.g. water voles Arvicola amphibius), fish (14%), and birds or amphibians (6%). A total of 266 intact plant seeds were recovered from 71% of the pellets, representing 50 taxa from 17 plant families, including the alien Cotula coronopifolia. The cumulative number of plant species dispersed was lower at higher latitudes. Eight plant species recorded had not previously been recorded as dispersed via waterbirds, and only three species have an endozoochorous dispersal syndrome. Plant taxa were dominated by Caryophyllaceae, Cyperaceae, Juncaceae, and Poaceae, with 24 species from the littoral zone (Ellenberg moisture values of 7–12) and 21 terrestrial species (Ellenberg moisture values of 4–6). Intact invertebrate propagules were found in 30% of the pellets, dominated by Cladocera (Daphniidae) and Bryozoa (including the alien Plumatella casmiana). Our results demonstrate that grey herons disperse plant seeds and aquatic invertebrates widely in north‐western Europe. Herons regurgitate pellets that contain plant and invertebrate propagules from both aquatic or terrestrial habitats, for which secondary dispersal via ingestion along with prey is the likely underlying mechanism (i.e. propagules either attached to or in the digestive systems of the various prey). Our findings showcase the potential of predatory waterbirds as vectors of plants and invertebrates, and how they may facilitate connectivity between freshwater and terrestrial habitats.

Recent field data suggest that migratory gulls disperse many rice field weeds by gut passage (endozoochory), most of which are dry fruited and widely assumed to have no long-distance dispersal mechanisms, except via human activity. We investigated this mechanism with a feeding experiment, in which seeds of five common rice field weeds (in order of increasing seed size: Juncus bufonius, Cyperus difformis, Polypogon monspeliensis, Amaranthus retroflexus, and the fleshy-fruited Solanum nigrum) were fed to seven individuals of lesser black-backed gulls Larus fuscus held in captivity. We quantified seed survival after collecting faeces at intervals for 33 h after ingestion, then extracting intact seeds and running germination tests, which were also conducted for control seeds. All five species showed high seed survival after gut passage, of >70%. Gut retention times averaged 2–4 h, but maxima exceeded 23 h for all species. Germinability after gut passage was 16–54%, and gut passage accelerated germination in J. bufonius and S. nigrum, but slowed it down in the other species. All species had lower germinability after gut passage compared to control seeds (likely due to stratification prior to the experiment), but the loss of germinability was higher in smaller seeds. There was no evidence that the different dispersal syndromes assigned to the five species (endozoochory, epizoochory or barochory) had any influence on our results. In contrast, mean gut retention time was strongly and positively related to seed size, likely because small seeds pass more quickly from the gizzard into the intestines. Non-classical endozoochory of dry-fruited seeds by waterbirds is a major but overlooked mechanism for potential long-distance dispersal, and more research into this process is likely essential for effective weed management.

The expansion of alien plant species is of global concern, yet our understanding of their dispersal mechanisms is limited. Here we address the potential of alien plant seeds to disperse via ingestion, transport and egestion in waterfowl (endozoochory). Based on their general rapid expansions, we expected alien plant species to have several advantages for endozoochory compared to native plant species. We hypothesised that seeds of alien species would have higher passage rate, longer gut retention times, higher germinability after gut passage and shorter time‐to‐germination after egestion by waterfowl. In order to test our hypotheses, we compared the endozoochorous dispersal ability of six pairs of congeneric alien and native wetland plant species in a feeding experiment with mallards (Anas platyrhynchos). We focused on differences in seed survival, gut retention time, germinability and time‐to‐germination. In the analyses we corrected for seed shape and volume as these seed traits are known to have important effects. With gut passage, alien species had higher passage rates and germinated slower, whereas native species had shorter retention times and greater germinability. Controlling for seed traits did not alter these conclusions, but seed traits affected all aspects of the endozoochory process. This suggests that alien species may have particular traits correlated with a higher endozoochory potential. Among control seeds, alien seeds germinated faster and their germinability was higher than natives. Seed traits explained differences in germinability and time‐to‐germination in control seeds. Seeds of alien plant species have traits that correlate with successful endozoochory. This may provide alien species with a competitive advantage over native plant species by ensuring higher endozoochory rates in new environments, potentially enabling their rapid expansions. Our study underlines the important role of seed traits in the endozoochory potential of alien and native plant species, notably through their influence on retention time and germination.