Why must offspring of birds develop an egg with a hard structure

The hard structure is to protect the offspring from harm. If you seen a mother bird before they often sit on their eggs to keep the baby birds warm and safe, that is why the eggs have a hard structure, so the mother bird doesn't accidentally crush the baby birds. 
Plus I believe that when the baby birds are about to hatch they will peck at the egg cracking it slightly signally the mother bird to get off of the eggs and watch their babies being born. 

Hope this helped :)
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