My duck eggs seem much harder to crack open than my chicken eggs...is this normal? Or should I lower the oyster shell content? I mix it in (per the reccomendation on the bag)with the pellets and feed Dumor which has a higher content of calcium than the Layena.(those are the two brands most readily available) My ducks are very picky eaters and usually will only eat either tomatoes,peas,strawberries, lettuce or grass clippings (dandelion greens,clovers,etc.).