DO you have a way to make a small fire w/o causing trouble? and a metal coffee can, "chimney starter" or similar?
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Basically, you need to cook and dry the oyster shell for sanitation. Something like this is all you need. A few sheets of paper on the bottom to get things started, some charcoal briquet or even sticks in the middle to form coals, then wrap some oyster shells in aluminum foil and toss 'em on top - or skip the aluminum foil and accept a little carbon on the outside. It doesn't need to be stupid hot, just an average shell temp above boiling for some moderate period of time - 5-15 minutes.
Once things are cool, crush with a heavy object (eye protection, gloves. Safety third!!!!) and feed to your birds.
You could even make a solar oven for doing this. But a converted metal can to form a chimney starter is probably easiest.