Last week I visited a wonderful wildlife garden near Bristol where the owners had been extremely successful in developing colonies of red mason bees Osmia rufa, by providing tubes for them to nest in on the south-facing window ledges of their house.
Each one of these individual tubes contains several nest cells, each with an egg provisioned with pollen and sealed with mud.
The other key element in the garden was plenty of early-flowering pollen sources for the bees - hellebores, pulmonarias and the like. The activity in this colony had to be seen to be believed - scores of bees coming and going all day long. The benefit for the garden owners was that these insects are very efficient pollinators of their soft fruit bushes and fruit trees in spring.
The excellent Bees,Wasps and Ants Recording Society (BWARS) provides a very useful downloadable information sheet on these delightful insects.