We have three bird nesting boxes on one of our trees in the back garden. Two of them are quite old but the best one is a new one, it was a Christmas present a couple of years ago. We had blue tits nesting there last year – it must have been a very comfortable experience for them as I have a bundle of wool insulation from a food pack hanging underneath it.
I was looking out for them again this year but nothing was happening. One day, however, I noticed what looked like big flies crawling around the opening. To my surprise, after a few days, it developed into a large crowd of bee-like creatures.
There are quite a few of them there, the whole box must be packed with them. It is fascinating to watch them. I got more information about them, the tree bumblebees are very interesting.