The nest box at the lower left here is what I got them:

One of the Javas likes his box and plops down in the door and snoozes or sings. But he'll come out and perch on my finger if I present it to him.
The other little guy, however, when he goes in his nest box, he stays there, and he becomes possessed, insane. I cannot get him to come out. He gapes and jabs at me violently if my hand gets anywhere near the door, and the only way I can get him out is to let him get a good grip on my finger with his beak and then pull my finger back. Once out of the nest box, he's fine again, a little pussycat. It's a Jekyll and Hyde thing.
I'm probably fooling around with some deep-seated instinctive behavior patterns. Is this something to worry about. I'd hate to take away the nest boxes that they seem to love so much.