Oh, bummer. I was going to say if you cut a grape tomato just right you can slip a small calcium tablet inside and feed it to Faith like a pill pocket. I haven't found anything else that works as well. My vet suggested banana or oatmeal. I wonder about actual grapes, or any other duck safe human food.
I got powdered avian/reptile calcium from Amazon before and put mealworms in a baggie and "shake and baked" the calcium powder on them. My ducks didn't think they were as delicious but they still ate them. I've done the shake and bake method with live crickets I bought at Petco, too. I think half the crickets got away before my ducks could catch them. But I was desperate for calcium so I didn't care.
My vet prefers I deliver a specific dose down the throat with a syringe. I prefer to maintain my loving relationship where my ducks view me as the bringer of deliciousness.
I've also found that my ducks are super picky about their oyster shells and will only eat grey shaved oyster shells and not the white rock kind.