A week or so ago I discovered that one of my two hives had died, unexpectedly. I went out today with my beekeeping mentor to investigate, and made some very interesting discoveries.
First, there were very few bees in the dead hive--maybe, oh, 300-400 tops, all dead (which sounds like a lot, but there were probably 20,000 at least when I put this hive to bed in the early winter). There were some signs of diseases carried by mites in the carcasses. The hive was about 1/2 full of honey stores. In a little crawl space under the screened bottom of the hive we found a mouse, very happy and alive in his little nest.
The other hive, which was quite weak when I put them to bed, is now overflowing with a massive colony, and almost out of winter stores.
My beekeeper friend suspects, as do I, that the bees in the now-empty hive absconded, perhaps because mites had weakened them and they panicked, and/or because the mouse kept them in a constant state of agitation. It seems most of them just up and moved in with the other hive, adding to their number, assimilated by their Borg Queen. They either abandoned their own queen, or she took off with some of the colony for parts unknown (and probably died, this being winter in New England, albeit a sham winter at best).
With my friend's help and expertise, I've now "checkerboarded" the two hives together, adding the honey stores from the dead hive to the living one, and creating a giant 4-story monolith. They're very strong, even with some sign of mite diseases, and will most likely make it through til Spring.
Lesson: Bees are weird. No one really knows why they do what they do.