Although round nesting holes are preferred by mason bee, they will use all kinds of shapes and sizes of holes, including these routered holes. these nests are created by routering grooves in pieces of wood, which are stacked and thus creating the nesting holes. colored markers to help mason bees find their nest can easily be done on wood. some of the ‘unused’ nesting holes are actually used by the bees for their overnight stay. for this reason it is always best to have more nesting holes then the number of female mason bees that are released.