I have always purchased the small (3 lb?) bags of oyster shell from Purina. Last trip to the feed store they had 50 Lb bags for a lot cheaper. The Purina oyster shell is white, the 50 lb are more natural oyster shell color. I assume that is not significant - but thought I would check on here. Says on bag it is a good source of calcium, but nothing specific pertaining to ducks. And they don't seem to want to go near the stuff - new and strange or did I make a mistake getting the big cheap bag?