The effectiveness of a seed disperser depends on the quantity of seed dispersed and the quality of dispersal provided to each seed. Relationships between the quantity and quality components and their dependence on characteristics of the dispersers remain largely uninvestigated. 2 The effectiveness of different waterfowl species at dispersing seeds of Ruppia maritima was evaluated in a wetland in south-west Spain. Droppings were collected during autumn and spring waterfowl migrations and the number of seeds ingested (estimated from seed fragments), undigested and viable in germination trials were determined. 3 Ingestion by waterfowl enhanced the rate of germination and, for several duck species, it also had a positive effect on germinability. Both the presence of seeds in the diet and the effects of gut passage showed high interspecific and temporal variance. Some of the interspecific variation in dispersal quality was related to gut structure: species with heavier gizzards destroyed a higher proportion of seeds and undigested seeds ingested by species with more grit in the gizzard germinated better 4 In the waterfowl community studied, the quantity and quality components of seed dispersal effectiveness were positively correlated across species.