Seasonal dietary shifts in waterbirds modify the dispersal potential for invertebrate resting eggs

Waterbirds play a crucial role in endozoochory, facilitating the dispersal of invertebrates through the gut passage of resting eggs. They adjust their digestive processes and tract morphology in response to seasonal dietary shifts, which likely have an impact on the dispersal of invertebrate resting eggs. However, relevant research in this area remains scarce. Captive feeding experiments were carried out with mallards to evaluate the influence of diet alteration on the dispersal potential of zooplankton resting eggs and the digestive organ size of waterbirds. Mallards were provided with either an animal-based or a plant-based diet to mimic different food compositions during summer and winter. Resting eggs of five invertebrate species were fed to mallards prior to the initiation of diet treatment, and at one, two, and four months following the diet change. Eggs recovered from faeces within 48 h were counted and incubated. Gut morphology was compared after five months. Mallards consuming an animal-based diet exhibited significantly smaller gizzards and small intestines, and had a shorter mean gut retention time (x = 2.27 h) for resting eggs compared to those on a plant-based diet (x = 2.74 h). Diet treatment did not significantly affect the total number of retrieved eggs or their hatching rates. Few eggs were recovered from the digestive tract 3 h after feeding, and the remaining eggs were concentrated in the gizzard and small intestine. Egg retrieval following gut passage was highest in the cladoceran Daphnia pulex and lowest in the rotifer Brachionus calyciflorus. Diet treatment did not affect egg hatchability, which was lower than that of control eggs. This study indicates that diet change and digestive tract morphology can affect the waterbird-mediated dispersal distances for resting eggs. Seasonal dietary shifts towards increased plant consumption during autumn and winter may extend the retention time of invertebrate eggs, enabling waterbirds to transport propagules to more remote habitats.