Chuck,
  There was an article on how laboriosa overwinter in the valleys, and with clu
ster temperature actually lower than ambient in the day, by Underwood.
I think it is on Natural History.  sometime in 1991.
   I also remember reading about the same bee in Nat His. during the meeting of
 New Orleans, so maybe Dec 1990, or Jan 91, but I am not sure if that is the on
e about migration during winter.