SO WHAT HAPPENED??
Feb. 1st - 5th -2, -18, -4, -19 degrees F in the mornings with highs about 10 degrees.
We had very little snow at this time so not the normal protection. Our heavy snow didn't come until late Feb. and March. The best I can come up with is they were not able to move to a new food source because of the extreme cold - they died before the weather warmed. Note that the 2nd supper was empty in the middle - this is most likely where they died. HOWEVER
There are problems with this theory. I would have expected more dead bees left in the cells between the frames of the 2nd super. Two of my UP old time beekeeper colleagues think it may have been a mite problem (tracheal) or a combination of the cold, mites, and possibly a virus. If anyone has suggestions, I am all ears.