Bluebird in the Chickadee's Nest

On 28 April 2018 I photographed an anomaly in the bluebird box: One bluebird egg, and the others, much smaller, white with copper spots. Bluebirds had built the nest -- I watched them. But after one egg (usually they lay five) the bluebirds split the scene or were evicted and a black-capped chickadee took over. I confirmed it was a chickadee when I saw the mom flee the box as I approached, and bluebirds don't have downy white feathers to line their nests with.

I check the bluebird box about every three weeks to make sure all is healthy and clean. (I've found snakes in there, bees, a bat, etc.) I thought, surely the chickadee mother would ignore the bluebird egg or starve the bluebird baby, or peck it to death,  if it survived. But on 19 May I opened the box again, thinking I'd surely find at least one dead baby bird, and maybe all of them, considering. I found a nest full of life.

Here's the egg photo, what it looked like three weeks ago. We might have lost some baby chickadees, but gained a bluebird:

Surprise

Whippoorwills and owls are now calling at night, and occasionally in daylight I inspect the bluebird box to make sure all is well (and that snakes and bats haven't invaded it). Removing the nest, I noticed with approval that it was lined with extra white fluff, unusual considering that bluebirds have no white feathers, and I was not expecting, at that moment, to see eggs in the nest, but I did. And was very surprised.

A bird that wasn't a bluebird, more like a chickadee, shot out from the box as I approached, and the rust-spotted ones must be her eggs; they match with other photos of chickadee eggs. Apparently the bluebird couple was evicted after producing one egg, and now their blue egg shares the nest with the other, smaller, spotted eggs. Never seen this before. Put it all back as it was. After the chickadees are hatched and raised I can clean out the box so the bluebirds will return and breed there.