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There is no need to mix it into the food, though some do. Grit is taken as needed, and it's function is such that birds only need it now and then, not every day. The bird takes in some grit and the grit remains in the gizzard where it is used to grind food -- eventually pieces of grit become swept up in food passing through the rest of the digestive system and pass out through waste, and the bird tops off their grit supply again by taking in more grit. Each bird may only take a few pieces here and there - so you aren't going to see a regular emptying of the grit supply that requires constant topping off.
