It has been fascinating to watch the great tits set about building the nest from scratch. The nest is constructed mainly of spaghnum moss brought in strand by strand with some downy feathers and hair or wool. Although the nest looks woven it is actually made by pushing the strands to the side. As the volume builds up the central concave part of the nest appears simply because it is where the birds sit to push the moss towards the sides. The male spends a large part of his time in the nest box pecking fiercely at the wood around the entrance (as above). It doesn't appear to be making the entrance larger but possibly it is making it smoother. They only have one brood a year so success is absolutely critical to each bird and hence they go to a lot of time and effort to make the nest secure and secret. I think they would both be deeply shocked if they knew how closely they were being watched deep in their own home.