Well, technically that’s true. Without analyzing a fertilized egg, we don’t know with certainty what the result will be.

For example, a woman could give birth to an albino without knowing before birth. Albinism is a mutation in the melanin production gene. The mutation forms in-utero. The equivalent to an in-utero mutation in an oviparous (egg-laying) animal would occur inside the egg.

So the direct ancestor of the chicken laid an egg that mutated into the first chicken egg, then the first chicken hatched from it.