I have seen Eastern Bluebirds visiting the Nest boxes in my yard throughout the month of March, but until a day ago, investigation behavior was all I had observed.

I noticed a female bluebird visiting the nest box but she wasn't entering the box. She repeatedly fluttered around the entrance and landed on top peering down toward the entrance.  Her mate was on a limb above, singing.
When she did finally enter, I could hear screeching and the male bluebird flew down from his perch wing-waving,  He was clearly prepared to come to her aid, but out flew a Carolina Chickadee followed by the female bluebird who landed on the top of the nest box.
I thought the matter was settled at that point, but apparently both the male and female chickadees were in the box when the female bluebird arrived.  The female bluebird continued to flutter around the box entrance and sat on the top of the box for a long while watching and listening, with intermittent peering inside.
In past years, I have had a bluebird build her nest on top of a nearly completed chickadee nest. Though, very brave-hearted, I don't think a tiny chickadee stands a chance when a bluebird decides she wants the nest box.

Ten minutes or so of the bluebird's repeated peering into the entrance and call exchanges and the female chickadee finally departed.  I later checked the box and found that she had the floor of the box nearly covered in moss, a first soft layer for her nest in progress.
Once the chickadee left the box, the male and female bluebirds, in turn, went inside to investigate. The female of the pair will select the nest box, though the male is very active in finding boxes and escorting her to them to take a look.
Now it remains to be seen whether this pair of bluebirds will actually nest in the box. Since they've driven the chickadees away, I'm hoping that means they're staying.

More seasons with Eastern Bluebirds on this blog:  Eastern Bluebird Family
Videos of an Eastern Bluebird family
Carolina Chickadee posts on this blog.
Eastern Bluebird art