We have a bee viewing box attached in a very sunny spot on the side of the hide. The front is removable with glass tubes so you can see whats going on. And, this year, this is whats going on:
These are the eggs of the Red Mason Bee (Osmia bicornis) – one of the bees that use tunnels in wood to lay its eggs and so would be an expected user of insect hotels. They build mud plugs and, in-between these, put a pile of pollen and lay a white egg on the pollen. Once the tunnel is full, they build a thick mud door. Here are some more photos of the three tunnels that are built so far:
You cannot help but have respect for this wonderful bee and the amount of work that is involved in all of this. Here she is working on the door of one of her tunnels

And this next photo shows a blob of wet mud that she deposited before she spread it over the door.
The Schwegler box that we have is shown below (only the bottom right tunnel has been completed in this photo)
However, I now understand that bee experts do not approve of these boxes – the glass viewing tubes are not porous leading to fungal problems within the nest. I think it is too late to do anything about this now since four tunnels are already being worked on by the bees, but we will retire it as soon as we can and have just bought a replacement box that gets round this problem and can be cleaned of lingering parasites. Looking forward to that arriving and seeing how its constructed.
And talking of parasites, also hanging around the box is this little lady:

Although she is tiny and doesn’t ever keep still – so frustrating to try to photograph – she is the most beautiful thing. I attach below an internet photo to do her justice:
However, her lifecycle is not as admirable as her appearance. She is busily investigating all the holes in the wood of the shed to try to find mason bee nests into which she will lay her eggs. These will hatch and feed on the mason bee eggs and larvae as they grow.
The more I learn of the complicated interactions between these different insects, the more fascinating it becomes. More time is going to be spent hanging around the orchard with a camera.