I have (or had) two top bar hives this past summer that I'm overwintering now. I started my original hive from a package and then ordered a new queen and made a split later. Oddly enough my second hive seems more populated and robust going into winter than my original.
Either way, over the past months I've seemed to notice less and less activity from my original hive until the point that I was pretty confident that it was going to die out. My second hive has been perfect this whole time. Every day we have that's a little warmer than normal they are working away and bringing in bits of pollen. I'll sometimes look through the window on the side of the hive and see them balled up together to stay warm on cooler days.
Well, last night while I was out to feed my dog I decided to take a quick look through the windows of both hives, really just to see if the original still had any live bees balled together. It was 100% empty! The only bees I could see anywhere where just a few here and there dead on the bottom or side of comb. So then I go the the second hive and it's packed full of bees! It was cold last night too, and I know the usual size of the ball they are packed into in that hive. This was about twice the size as it normally is.
The only thing I can really assume at this point is that the queen died out some way in the original hive and the workers have slowly migrated over to the other hive to join it.... Which I'm perfectly fine with. It will make it an easier split this spring with a nice population of established bees already in this hive. I just didn't know that they could/would do something like that. It may be perfectly normal and common but I have never heard of it before. Anyone had this happen before?