I think they might have gotten confused and swarmed too late in the season, not leaving enough population to keep their temperatures up. Also, we are using a hive design which is a bit different from the Langstroth hive - it's called a "top bar" hive, which was originally designed in Kenya. It is a very simple, one-layer design (brood comb and honey comb are all in the same level). Because it was designed in a tropical region and does not have multiple levels, I'm thinking the insulation is not quite as good as it is in the the Langstroths. However, top bar hives *are* used successfully in colder climates, so I think with proper placement the hive could be kept warmer (for example, placing it near a stone wall that gets plenty of sunlight and stops the wind from hitting the hive).