Digging is natural for dogs. Being of one family with wolves they dig for mice, because mice, rabbits and other small rodents are the main food for wolves ( not a deer as many people think). I have watched a film about wolves in Canada, they can detect a mouse under 4 metres deep snow. There must be something in the ground, if your dog digs so insistantly. But this behaviour stops itself with age and experience, though your dog may start making holes in different places before then. She is bored.
In order to teach her out of chicken hunt you have to train her that chickens are a worthless prey, and they are something to ignore by demonstrating ignorance yourself and redirecting her to a ball. You would need chickens living in your garden for that. Put up a fence, it is easier than training your dog with considerable experience of killing them.