Nice article in the gardening section of the local newspaper today about how to provide nests for native solitary, non-social, bees. Basically, they consist of aged wood with multiple holes drilled in it, 3/16 to 5/16-inch diameter, variety for different species, 4 to 6 inches deep, shaded from intense sun. Mud plugs indicate they have been used. Sounds easy enough. (
Cal Berkeley advice for Northern California nest makers)
They gave some astonishing figure for how much more effective at pollinating were the native bees over honey bees, as I recall at least a couple of orders of magnitude -- because honey bees collect pollen in sacs to bring back to the hive, while "good-for-nothing" native bees just get dusted all over with pollen as they go from flower to flower.
I don't have many flowers, thanks to shade trees, but I'll sure stick a bunch of nests up around the back yard. A few logs from the wood pile, a drill, and 10 minutes of effort and I'll become a bee-landlord.