One of the keys for mason bees is to provide sufficient food supply throughout the season, especially during the early spring. Bumblebees need this food source in early spring as well. I have tried mason bee nest but with out much recurring success. I do keep buying them as there is plenty of natural habitat for them and am trying to increase a local population
One of the practices of encouraging bumblebees is to keep stacks of limbs scattered throughout the property. (
http://bumblebeeconservation.org/about-bees/habitats/)
Bumblebees, mason bees and honey bees are all important for the garden as each has a niche that they work in.