A t this time of year as you set out your mason bee cocoons, dont forget to provide a ready source of mud. This video shows a little trick I discovered where they seem to prefer mud tunnels to gather their mud for the tubes. This makes sense as there is less chance of predation by birds when picking up mud when the bees are out of sight.
Dig a small 5″ deep trench within a few metres of your bee houses, and bore horizontal holes into the side of it. Put a bucket of water in the trench when it looks dry.