Cedar bee houses. Designed to accomodate Manitoba Mound and Leafcutting bees. We have them at our camp and they work very well. 
 
"Around the world, pollinator insects are under threat from … parasites and pathogens, pesticide use, habitat loss and climate change, especially in urban areas. Bees play a critical role in ecosystems, including pollination of many food crops we rely on. In urban areas, creating artificial habitats like bee houses that are appealing to the public, easy to use and attractive to bees can help support healthy and resilient pollinator communities."
                                                                               (Robert Currie, an entomology professor at the U of M)