Greg,
What I was trying to explain in my 1st post. Bees have 2 options for survival under cold conditions. One is behavioural (clustering) (emergency) and the other is to "naturally" use their hive enclosure (primary) to protect them from the environment. The enclosure can protect the bees from extreme cold if the cavity can provide enough thermal protection (R-Value).
There are 3 key variables:
- Size of cluster (heat generation - Passive or Active)
- Size of Enclosure (interior volume - how much space that needs to be heated)
- Enclosure R-Value (capacity to retain heat - reduce thermal loss)
- Other variables: quantity of honey (thermal mass), type of ventilation, leaky seams...
Here are 2 scenarios:
1) Wooden hive, in a double brood format, with 18 frames of bees (Emergency survival mode driven by cluster behaviour)
Bees will passively release heat and enclosure will retain some of this heat. As temperatures drop, heat loss will be larger than passive heat released from the bees. At some point they will need to contract into a cluster to slow down this heat loss. As the temperature continues to drop, some of the bees will need to actively generate heat to keep themselves and the cluster warm. This behaviour seems to be driven by the bee's local temperature. (Self organizing cluster). Each bee's feedback system adjust to local conditions.
Variations to this scenario - a small volume with large number of bees would likely cluster at slightly cooler Tout due ratio of bees to volume. A small cluster would need to cluster at higher Ts.
2) Insulated hives (Primary survival driven by enclosure properties)
The high R value means that the bees will be at resting metabolic rate at lower temperatures before they need to cluster and start actively generating heat to maintain survivable conditions.
Active metabolic rate (shivering) >> Passive metabolic rate (no shivering = resting metabolic rate) (see chart) and look up the referenced paper. Also read up on self organizing cluster behaviour - it is very fascinating.
