Today Chris and I cleaned out the blue bird boxes.
The second nest was constructed by Tree Swallows, not Bluebirds. You can tell because it is lined with feathers. Tree Swallows like Bluebird houses, and they love white feathers. I have seen them playing with feathers, dropping them in the air, then swooping down to catch it. Drop, swoop, repeat.
The boxes are now clean and ready for birds who want to have a cozy spot to raise a brood. I so hope some do!