Bumble bees will dig into loose earth usually under leaf litter ("the roof") in a well sheltered spot, under a tree or the face of a steep hill. Occasionally, to aid herself in excavating a hibernaculum a bumble bee gyne will rapidly vibrate the substrate with her thoracic flight muscles to loosen soil. She literally digs in for the winter. There is no true cavity, just the female surrounded by an inch or so of loose soil. A gyne accumulates alot of non-metabolizing fat reserves that will protect her vulnerable tissues from extreme cold. Snow is also a wonderful insulation, especially in Canada where it remains on the ground. Cheers, Blair Blair Sampson [log in to unmask]