Some hives also seemed better adapted to the cold. I was so busy last winter (with my kids) that I didn't do anything to help my bees overwinter, and ended up losing nearly a third of my bees to cold, wet, and starvation, which was really sad in spring and meant I spent a good part of this last year playing catch-up instead of making honey. The "upshot" was that the bees I was left with were all very cold-hardy strains. Some of my hives that seem weaker (only in a single box) are actually doing fine - there are nectar and pollen sources within 50m of the hives and every single hive had bees flying.
I went out at around 10 AM on a Saturday morning to check ten of my hives that are hiding under some gums (blue gum mostly I think, but I'm terrible at Eucalyptus sp. identification). Middle of winter in Mount Gambier usually means hard rain, lows of 4-6°C at night and highs of 10-14°C during the day. The problem with a temperature profile like this is that the bees are often too warm to ball up properly during the day but the rain keeps them from flying, so they burn through their honey fast. I discovered last year that hive survival seemed related to how well they were insulated, how big the colony was going into winter, and how much honey I left them. Those with a quilt box did a lot better than those without, and those that were off the ground did better than those on the ground.
My bees are a real mixed bag in honey production, temperament, and cleanliness, and I wonder if that's a result of the number of other beekeepers around where I live. My bees in my more remote/non-urban sites seem better than my rural/urban bees. More genetic diversity = better division of labour in the hives? The more I read, the more I realise that bee genetics are poorly understood even by (especially?) the experts, and I'm still very much a novice!