We have had nesting bluebirds in our box for the past 15 years. About 2-3 weeks ago the male and female began coming and going from the box with food, and at one point I saw the female fly out with a fecal sack, so was quite sure babies had hatched. Just now I took some mealworms out, and both parents came and went with worms. But the mom took a mouthful up into a tree nearby. I was curious, so looked into the box and nothing was there! Not even a nest! Can anyone help solve this mystery??