Protecting Mallard Duck Babies from Stray Cat?

I have a mother Mallard that has laid 9 eggs under a bush in my backyard, which is a very small, fully fenced in yard. There is no male Mallard in sight. My problem is that there is a feral cat that keeps attacking the mother Mallard and I'm afraid once the eggs are hatched the cat will kill the babies. I know it's illegal to move a nest, but the babies won't be able to get out of my yard until they can fly. Any suggestions what do do?

By Carol