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: