Rich, we had this happen last year, the mama bluebird disappeared, along with two of the four hatchlings. We had no idea what happened. For about a day, the papa bluebird kept away, and the babies got seriously dehydrated. They looked about to give up the ghost in the nest. I was calling around frantically to wildlife rehabilitation hospitals (btw learned about the stupiest law ever, where you need a special “federal license” to handle migratory birds, and so no one would or could help–even though they knew how! Case of government regs meaning right but doing wrong.) ANYWAY, just when I arrived to take the nest to a place where they said they could try to keep them alive, I saw papa bluebird nearby. I felt I couldn’t take a nest if he was there. Instead, we loaded on the mealworms, just as you are doing–and darned if papa didn’t swing into action and bring those babies back. It was an amazing sight, watching him do it all on his own. Finally, they fledged. So keep feeding! It really helps! They can do it! (with a little help from their friends.)