I wouldn’t sit on it if I were you. The reason most birds can get away with it because they have feathers and a brood patch. In this way, they can safely sit on the egg and provide the necessary heat for incubation without the risk of it breaking from their weight.
In your case, you’re better off having the egg wrapped up in cloth or placing it near something that keeps it warm. If you’re going with fire as a source of heat, just remember not to put the egg too close to it or you'll risk having the embryo boiled to death instead.
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I wouldn’t sit on it if I were you. The reason most birds can get away with it because they have feathers and a brood patch. In this way, they can safely sit on the egg and provide the necessary heat for incubation without the risk of it breaking from their weight.
In your case, you’re better off having the egg wrapped up in cloth or placing it near something that keeps it warm. If you’re going with fire as a source of heat, just remember not to put the egg too close to it or you'll risk having the embryo boiled to death instead.