Its the first time I've had Bluetits reach the egg laying stage. For the last three years I've had them build a nest in the same box then for some reason abandon it. I thought it maybe because the 25mm entrance hole was too small for the female to get through if she put on weight developing her eggs, that was my theory anyway. When I made this box I installed two small double glazed windows on either side of the box made from clear plastic and a hardwood frame. Why did I do that, imagine you are a bird approaching the nest box in bright sunlight, then you block out all daylight when you squeeze through the entrance, you can't possibly see where your chicks are or where you'll land unless you have a window. Whether you think its a good idea or I'm totally bonkers feel free to install windows in your next box as I wont be taking out a patent.