There is no practical way to incubate the eggs – even if they were hatched, something (parents or other birds) needs to show them how to hunt for food & stay away from predators. There are song bird rehabbers which sometimes will try to save young ones, but they aren’t always able to be successful. I had 4 eggs hatch one time after mama died or was killed, after they had no incubation for 2 or 3 days! Papa was still around but would not feed the babies because he knew they needed mama for the first few days of life. I contacted a rehabber in Kansas City area but she did not give me much hope for them and they perished. They are now buried in our garden. Just clean out box & hope for a nest – good luck. And yes, put up a proper baffle & keep box a good distance from fences or trees where predator can climb or jump, as David says.