Learning from dead hives

At the bee club meeting last week, we looked at two people's dead hives to see what happened to the bees. along with one of mine which had the classic "sugar crystals" in the bottoms of the cells indicative of death by varroa mite.  These are actually uric acid crystals left from lots of varroa peeing in the hive.   One of the other hives probably died of starvation, as shown by the tiny dead cluster not in contact with honey, and the other tiny cluster died in contact with honey, but bunches of the bees dies head-in to cells.  If you read  the internet, this is the sign of death by starvation, but bees go head-in to cells normally, and for different reasons.  In reality, all three of these hives died of not enough bees.  One was too small to move to honey, one was able to move to honey, but not big enough to stay warm, and one mine just didn't have enough bees to survive varroa.  The one thing that all three hives had in common was a problem producing fat bees for winter. 
    Fat bees, what are they? Are they fatter?  Not visibly..  However, fat bees have more fat cells in their bodies because they haven't functioned as nurse bees.  This helps them live all winter, as opposed to summer bees that live  40 days, tops.