Digestive plasticity in Mallard ducks modulates dispersal probabilities of aquatic plants and crustaceans

1. The consequences of plastic responses of the avian digestive tract for the potential of birds to disperse other organisms remain largely uninvestigated. 2. To explore how a seasonal diet switch in Mallard (Anas platyrhynchos L.) influences their potential to disperse plants and invertebrates, we recorded the retention time of markers, following exposure to two diets of contrasting digestibility (trout chow vs seeds). 3. We then recorded the retrieval and germination of Fennel Pondweed (Potamogeton pectinatus L.) seeds and Brine Shrimp (Artemia franciscana Kellogg) cysts ingested by the same birds. 4. Gut passage rates of markers were increasingly longer in birds on the seed-based, high-fibre diet and shorter in birds on the animal-based, low-fibre one. 5. Propagule digestibility, and thus survival to gut passage, differed between diet groups, with more seeds and fewer cysts retrieved from ducks on the animal-based diet. Germination decreased with retention time, but was not affected by diet. 6. Differences in passage rates of markers but not of seeds and cysts suggest no change in dispersal distances of plants and invertebrates between seasons, while differences in digestibility would affect the numbers of propagules dispersed. [KEYWORDS: Artemia franciscana ; diet switch ; endozoochorous dispersal ; Potamogeton pectinatus]