this is what i'd picked up
"Brood rearing starts soon after the winter solstice – initiated by increasing day length – but most colonies do not begin to increase until late March or April. The population decreases as bees die from old age – and increases as brood develops. "
wrt scientific papers:
Journal of Theoretical Biology
Volume 128, Issue 3, 7 October 1987, Pages 329-337
Why honeybees rear brood in winter. A theoretical study of the water conditions in the winter cluster of the honeybee, Apis mellifera
Stig W. Omholta
aN-6530 Bruhagen, Norway
A model is presented describing the water conditions in the winter cluster of the honeybee, Apis mellifera. It is based on a specific view of how the thermoregulation of the winter cluster is accomplished. It assumes that the apparently coordinated thermoregulatory responses of the broodless winter cluster are results of the individuals' acting behaviourally and physiologically to regulate their own body temperature. The results include predictions about radial distribution of water evaporation and accumulation per bee, and cluster humidity, as a function of ambient temperature. The main conclusion is that the phenomenon of brood rearing in the winter cluster is likely to be a strategy which the bees have adopted in order to reduce their own water content when this has reached a specific level.