All 4 hives are double deep with Honey Run top covers (with 2" insulation) and upper and lower entrances. I decided to use entrance reducers turned to the smallest holes as mouse guards. Today it reached the upper 30's and some of the bees decided that it was a fly day. Toward the evening, I noted a pile of bees dead at the entrance to one of the hives. They looked like they were all bottled up trying to get into the lower entrance. After removing the entrance reducer, I found that the bottom board was basically a soild chunk of dead bees and ice completely blocking the entrance resulting in no air flow through the hive. All four colonies were the same. So before the sun set, I suited up, removed all the reducers and literally had to chisel the chunks of dead bee ice from the bottom boards through the bottom entrance. My assumption is that the normal accumulation of dead bees through the winter started restricting the air flow through the small lower entrance hole. Then the condensation started building up, dripping on and killing more bees and eventually forming the ice chunk on the bottom board. The bees that perished today probably flew out the upper entrance and couldn't get back in through the blocked lower entrance and froze. This is the first and last time I use an entrance reducer as a mouse guard. It might work in warmer winters, but not in Utah. Lesson learned. Hopefully the surviving bees will make it through.