However, we do not require it for nutrients after a while but rather we need it to farm germs that enter the embryo so it can function properly. We do gain nutrients out of it but it is not the core mechanism for fetuses to gain nutrients. We have evolved to have a better system of nourishing ourselves at a young age.
If you have sapient birds in your science fiction story then they probably have different bodies and a different nervous system (i.e brain) than our “regular” birds due to having to stay in the egg longer.
To any bird lovers being offended at this I admit that birds can be very intelligent. Who knows, maybe they already are sapient and we just don’t have the proof. But for them to be unusually science-fictionally relatable-to-humans type of intelligent, they would require a type of nourishing system at birth similar to ours. Have your bird eggs be alien to us: explain that in order to grow proper brains and neurons birds have evolved to have strange, pulsating eggs. Depending on how weird the story is maybe the parents must even feed the egg through a toothless mouth because it doesn’t gain nutrients from the yolk sac after a while.
Gross, I know. But science fiction realism is fun to think about, right? Anyone? Just me?