I agree that bees keep it only as warm as needed.  Southwick observed tightening of the cluster as it got colder.  The hairs actually interlace to produce an equivalent of feathers or fiberglas insulation.  

The cold bees moving in obviously can be observed, but I doubt anyone has  figured out how to watch core  bees in a tight cluster without disrupting the cluster.  No reason warm bees may not move outward.

One thing that Ed told me, don't put a comb full of honey on the outer edge of the hive body.  We all know bees tend  to not fill the outer or outermost combs.  Ed found that when it got really cold, the cluster moves over against an empty comb.  Their bodies pressed up against empty cells provided a bee created form of insulation board.

Jerry



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