I don't think an un-absorbed yolk qualifies as a birth defect. The only times I have heard of this happening is when a hatching chick is helped along. The yolk absorbs generally pretty close to the end of the hatching process, so even if a chick has pipped and started to zip it does not mean the chick is ready to be out of the shell yet! Wrapped in a damp paper towel and back in the bator is probably the best you can do. I wouldn't advise giving it anything to eat or drink yet. Good luck!