I once placed a (different breed) bantam egg under my MGB that was trying to go broody, just so she had something to sit on for a while. Every night I took her off and put her in her roost to keep her safe. On about day 10 of this I decided to crack the rotten eggs incase they exploded...
This one I watched hit the tree, only to see red show between the cracked shell. It was a chick; and it was alive! It was moving. It had feathers.
If I remember correctly it got down to about 17C at night, or a bit less. I was amazed that it survived so long getting so cold every night!
I wished I would have candled it first - but who would have thought...?