We have great tits that are using one of our nest boxes. It built its nest a about a month ago (or possibly still building). Over the past couple of days, every few minutes they will fly out and a couple minutes later will come back, and back out again and so on. This is both birds, not just one. Sometimes they’ll leave at the same time and then come back around the same time too. I have noticed that sometimes they’ll take some materials into the box but also sometimes they’ll take some out. What does this mean?