i've tried for five years to get bluebirds to nest in one of my boxes. This year they did and had 4 eggs hatch. The nestlings were just over a week old. I checked them two days ago and they were fine. This afternoon, after setting out some mealworms, I checked the box and all four were dead. There is no sign of disturbance. They were very pale brown in color. Is it possible the heat killed them? Is it too late for the adults to nest again?