Field studies have shown that waterbirds, especially members of the Anatidae family, are major vectors of dispersal for a broad range of plants whose propagules can survive gut passage. Widely adopted dispersal syndromes ignore this dispersal mechanism, and we currently have little understanding of what traits determine the potential of angiosperms for endozoochory by waterbirds. Results from previous experimental studies have been inconsistent as to how seed traits affect seed survival and retention time in the gut, and have failed to control for the influence of plant phylogeny. Using 13 angiosperm species from aquatic and terrestrial habitats representing nine families, we examined the effects of seed size, shape and hardness on the proportion of seeds surviving gut passage through mallards (Anas platyrhynchos), and their retention time within the gut. We compiled a molecular phylogeny for these species and controlled for the non-independence of taxa due to common descent in our analyses. Intact seeds from all 13 species were egested, but seed survival was strongly determined by partial effects of seed mass and hardness (wet load): species with seeds harder than expected from their size, and smaller than expected from their loading, had higher survival. Once phylogeny was controlled for, a positive partial effect of seed roundness on seed survival was also revealed. Species with seeds harder than expected from their size had a longer mean retention time, but no phylogenetic signal was found for retention time. Our study is the first to demonstrate that seed shape and phylogeny are important predictors of seed survival in the avian gut. Our results demonstrate the importance of controlling simultaneously for multiple traits, and that relating single traits (e.g. seed size) alone to seed survival or retention time is not a reliable way to detect important patterns, especially when phylogenetic effects are ignored.

We conducted three consecutive feeding trials. During each trial, each of the eight individual mallards was force-fed with 100 seeds of the small-seeded taxa and 50 seeds of the two larger-seeded species (Glycyrrhiza echinata, Sparganium erectum). In order to investigate how seed traits influenced seed survival and retention time, we measured the following five traits for seeds that had not been ingested by birds: hardness, thousand seed mass, volume, water permeability, shape, phylogeny.