Last winter was my first winter with bees. By using an observation window on cold winter days I could see the bees clustering directly under the bottom of the top bars. When the days got colder I put a 2 watt bulb inside a Styrofoam box (with no top) and fastened the Styrofoam box under the solid bottom board of my topbar hives. When the light bulb was on, the bees would be clustered near the bottom of the wax comb. If the light was off, they would be clustered at the top.
I know this isn't news to experienced beeks, but it sure convinced me that the bees will seek a warm part of the hive regardless of wither food is available or not. Since there wasn't any honey for food at the bottom of the combs, I only used the light bulb heat source on the coldest nights.
In retrospect, I was probably lucky they didn't get stranded away from a food source. This year, I've decided to let the bees do what they do best and survive on there own (or not ) as the case may be.