How to postpartum care for zebra finch?

You need to provide them with a proper nest and move the egg into, right away. Today if you can find a pet store open. Just ask for Finch nests. They look like little baskets and are pretty cheap. Hang it inside the cage and put the egg in it, along with proper nesting material, which you can also find at the store. The main thing is to be sure they are getting a good diet, which it does sound like they are. Good for you on that. But you also must make sure she has plenty of calcium so she can make good solid shells for her eggs. If she doesn't have it, her eggs will be so soft she can't p them. Then she will be egg bound and that is fatal if not recognized and dealt with immediately. Put a cuttle bone in the cage and you can also sc a little off and put it on their soft, moist foods. Beyond that, they are prolific breeders and they will take care of the rest. I hope this helps.