Did you know this about Bluebirds?
 

The white things your bluebirds are carrying out of their nest box are “fecal sacs.” Yup, you read that right. As the name implies, fecal sacs are little packages of baby bird poop. And the birds carry them away in their mouths!

Very nice picture but a little TMI:)

Just good housekeeping. They also dispose of the empty shells and dead eggs. Can’t have the kids sitting on trash.
Occasionally they will have two broods in one season.

Felix Unger would get a kick out of that behavior.

Not the dead eggs.
We had 5 eggs this year, 4 hatched and when the parents evacuated the nest,
one egg was still there.

Seems like broken apart egg shells to me. We have had up to 3 sets of bluebirds in Spring and after the young birds fly away I empty the nest from the birdhouse and there are never any broken shells left in the house, so the bluebirds must be taking them out ( perhaps with some poop inside). BTW when the young birds are ready to come out the parents become very aggressive and if you approach the house they dive at you to chase you away. Imagine the guts those young birds must have when they come out of that birdhouse hole, jump and fly for the very first time.

Just clean out the old nest when the babies leave and someone will move right in.

We put our house for the first time this year. We had both house wrens and bluebirds interested and actually fighting over occupancy. Luckily, the bluebirds won and laid 5 eggs but only four hatched and the 5th egg eventually disappeared. All four fledged!
We had a second bluebird nest a few weeks ago with eggs. One day we peeped and the eggs were gone, with broken shells below. We have a guard on the post, so we know it wasn't a snake, raccoon etc. It had to be those darn house wrens! (They will do this!)
BTW, I've never had a bluebird parent attack me or desert their house when I "peep" in the house. Sometimes the mother is in there and is startled and sometimes I get fussed at. I don't peep when the babies are getting old however because I don't want them to fall out!

we're on our second eggs this year!