One year I saw 3 or 4 ducks with it, and geese get it the same way as the ducks. It is a parasite (sarcocystos) which apparently is not harmful, but I usually pass on eating it. The eggs end up in the water when the duck defecates, and when another duck feeds in the water, they can pick up the parasite. It is not thought to be detrimental to the duck, but I imagine it must sap some of their energy. So you decided to use it for your next sausage. I usually take about 50 ducks per year and I haven't seen it in a couple of years.