A more straightforward hypothesis is that brood rearing is initiated in response to the aging and dying of the fall-reared bees, and the 'perceived' need to maintain a minimum cluster size.  This would seem consistent with the lack of differences seen when light/dark is manipulated.  Ari's observation that the outdoor kept bees began brooding earlier than the indoor kept ones may be due in part to decreased longevity due to harsher weather outside.

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