When cleaning out the nest boxes, we found several mouse nests, but only two seemed to have any inhabitants. One had one mouse who jumped out when I opened the box. This one has four mice, and they stayed put. Their nest is made primarily of dog-strangling vine (DSV) seed fluff. It was interesting that when I opened the box and stuck my hand into the seed fluff to see if anything was there, I could feel an intense heat. Amazing that four little mice can generate such heat. The thick insulation of the seed fluff is clearly helping keep the heat in.