Unless you enlarge these pictures (click on the picture), it will be hard for you to tell that these are two different birds.
It actually took me a bit of looking as well. I thought it was the same bird, only it was fluffed up in one of the pictures.
I also was confused because I'd see a bird going into the birdhouse with nesting materials, only to see it leave with nesting material later.
Turns out, according to Cornell University, it's two male bluebirds fighting for the same nestbox.
In the bluebird world, the male builds the nest and then shows it to prospective females.
What's happening in my nestbox is, one male is bringing stuff in and the other is taking it out, and vise versa.
What neither bird seems to understand is, they aren't supposed to nest here at all. Every website and bird book I've looked at says they don't nest here. I guess they don't read? About three years ago, I saw bluebirds all summer. Their numbers have increased steadily since then. This year, I decided to put up a bluebird house that my father-in-law made for me, to see if I could draw a pair in.
I haven't seen any females looking at the box so far. Then again I don't spend all day hiding in a bunch of trees trying to get the perfect picture. I felt like a National Geographic photographer yesterday when I got these shots. I stood perfectly still for the longest time, waiting, waiting, waiting for the perfect shot. I came away with a couple keepers, hair full of oak pollen, probably a tick or two and some chiggers.
Hopefully, these two males will get things sorted out quickly. It's going to get too hot in that box pretty soon to raise babies.