Another Walk: Female Bluebird Comes and Goes

While the male bluebird was busy defending the nest, the female was equally busy tending the contents of the box. Note that the female is duller than the brilliantly blue make.
During this time, she was running a continuing cycle of going into the box, coming out, flying around and then back into the box. In preparing to enter the box, the female bluebird would look at me to make sure I was not coming closer.
She would then double check the other surroundings.
Then, she would focus her attention of what was in the box.
Then, into the box she would go - head in first.
Then, she would get all but the last of her tail in. Once in, she took nearly a minute, then turned around and peeked out to check the surroundings.
So what was in the box? Could be the female building the nest, or perhaps she was tending eggs or even hatchlings. At the next opportunity, I will open the box and let you know.