These tiny mason bees make nests inside wood cavities dug out by beetles. I stumbled upon this veritable high rise complex of tiny mason bees, nesting inside timber posts on an outdoor enclosure at Tracy Aviary.
These bees are solitary, so each female makes her own nest. The wood posts contain hundreds of these solitary bee nests. The bees are so small that the owls inside the enclosure as well as Aviary visitors are unaware that they exist! Not me though... ;)