Several days ago, I posted a note concerning cluster behavior. Approximately, I said "contrary to some opinion, studies have shown that individual bees in a cluster do not constantly rotate from the outside in and then outside again, but instead tend to stay put (either inside or outside)". I used this behavior as a point concerning my belief that "cold does not harm bees". More properly put, "reasonable cold does not harm bees".
Several list members asked me for my source of this information. I am reasonably certain it is contained in Tom Seeley’s wonderful book, The Wisdom of the Hive, but we recently moved an I cannot locate my copy to check. So I did the next best thing and asked Marla Spivak, head of the beekeeping program at the University of Minnesota, (her actual title is much more elegant) for the source. Her reply follows.
In her reply, Marla reminded me of another fascinating aspect of cluster behavior. That is, under really cold conditions the bees on the outside of the cluster pack themselves really tight, head in, and form a shell. This shell holds the heat in the cluster, rather than letting it readily disperse. This behavior lets the bees heat only the cluster rather than the entire hive. However, to generate heat the bees inside the cluster must have enough room to "shiver", so the inside of the cluster is loose with enough room for bees to readily walk around.
So, while Allen Dick is one of those experts who is not reticent about sharing information, his opinions from extensive observations and experience in one of the world's true ice boxes are not necessarily "truths" for most beekeepers.  (And Allen would be the first to make that statement.)  Question...does clustering behavior differ based on the degree of cold or the degrees of cold and humidity?   
Beekeepers can sometimes observe the shell behavior. In late autumn, a packed-out hive will often have bees’ cluster on the outside, where they are subject to cold rain. I have seen these clusters in the early morning, following a cold night rain. All that is visible is bodies and wings, perfectly aligned with one another so that rain flows off the surface as it would a turtle shell; quite a sight, and one of the great wonders of beekeeping.
The following is Marla’s message in its entirety.
"Hello Lloyd, Sorry for the delay in responding -- I have been out of town.

There is a good section on thermo-regulation in Mark Winston's book, "The Biology of the Honey Bee" Harvard University Press, 1987, pp. 116-119. He says that behaviors of individual bees in clusters was studied by Esch in 1960 (its an article in German). Workers in outer shell hang together motionlessly, but occasionally move into the center where they warm up in minutes. The may remain in the cluster for up to 12 hours, and then eventually rejoin the shell workers. Bees in center of cluster are loosely packed, and are the ones that generate heat. The shell bees have heads in and retain the heat.

Hope this helps -- Marla Spivak



Lloyd
Lloyd Spear, Owner of Ross Rounds, Inc.
Manufacturer of round comb honey equipment and Sundance pollen traps