a Mr Lagrange snip..
His theory is that our standard hives probably lose too much heat through the top as do our homes.

my comments..
in northern climate the equivalent problem in house construction is called ice daming.  basically heat and moisture from the bottom side of the enclosure rises and then forms ice on the inside of the structure due to the large differential in temperature between the inside and the outside of the house.  insulation and a solid membrane applied at the edge of the roof line are commonly installed in colder climates to limit this problem in building.  of course with a tree that is in anyway solid above this difference in temperature due to the mass of wood above would be minimal.  I would not be surprised (don't know how you could test the idea) that if ice daming occurs in a tree nesting site it most likely occurs towards the side of the nest and not the top.

not to dwell on the matter too much but basic physics does answer a lot of questions in the operation of this world.  

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