I use tree surgeons' waste for fuel so I always have a big stack of wood in waiting. I noticed a while back that some old trunk slices of chestnut had begun to rot away, so I stacked them to one side from the main pile.
they were used by bees as a nest site. This was unplanned by me, but it
suggests that if you can get some chestnut trunk or main branch slices at least
a foot (300 mm) diameter and about the same long, all you need to do is stand
them on edge in a shady place not too far from your bee-attracting flowers and
wait a couple of years until they start to soften at the centre.
They may well then become a bee condo, and for me they have worked vastly better than the silly bamboo things you get at considerable cost from garden centres.
I'm sure some other woods may serve as well, but I found chestnut the only wood the bees have used here.
If kept from getting saturated by rain, it rots
gracefully into a very soft dry spongy material that the bees probably find
easy to work. For that reason, these logs must be stood on edge.
If they're left cut face up to the rain they will go soggy and slimy instead of decaying gradually while remaining relatively dry. The very best solution is to build these pieces into a small relatively loose stack of assorted logs that will serve as home for other creatures as well.
If you put the bee logs near the bottom of the stack (but not in direct contact with soil), the other logs on top will also help to keep the rain off, and if they are of more durable woods such as ash, oak or some of the harder fruit woods such as plum, the roof they form will outlast the rooms of your bee condo.
Avoid eucalyptus though - I get a lot of it as firewood, but it smells strongly and I'm not sure the bees would like it.
You may get five years or more out of this wildlife hotel, but do keep it aside from your firewood, only renewing it when strictly necessary as all wild creatures object to being disturbed. If you're lucky someone or other will be in residence all year round.
Site owner's note: Pssst - Wild Soundscape has a lovely recording of a bumblebee flying by a microphone! Copy this link and paste it into your search box to have a listen (clicking the link opens a new window).