This study relates foraging behaviour and division of labour to a colony's energy state in wild-foraging bumblebee colonies whose energy stores had been experimentally manipulated. It examines: (1) colony-level effects of energy manipulation, to see if shifts in a colony's energy requirements produce shifts in the behaviour of its workers; and (2) the behaviour of individuals who changed tasks following manipulation and those that did not, to see if task specialization, based on learning, can explain differences between these groups. Individually marked bumblebees were observed at three colonies of each of three species (Bombus flavifrons, B. melanopygus, and B. occidentalis). Overall, colonies responded to changes in their energy stores, both by adjusting the relative sizes of the foraging and within-nest work-force, and by changing the behaviour of individual foragers. In response to energy manipulations, colonies allocated effort where demand was relatively greatest (foraging versus within-nest tasks), collected more of the resource that was in greater immediate demand (nectar versus pollen), and changed the rate at which each forager collected food (within-nest bout durations). Task switching behaviour was generally consistent with that expected by learning-constrained task specialization: (1) less efficient foragers switched to withinnest tasks when energy stores were enhanced; (2) workers who began foraging after energy depletion were less efficient than were continuing foragers and (3) workers who switched from pollen to nectar collection following energy depletion were less efficient foragers both before and after the manipulation. However, workers who switched from nectar to pollen collection were no less efficient at their old or new tasks than non-switching bees. In general, worker bumblebees show a flexible division of labour in the face of short-term changes in their colony's energy state. © 1992 The Association for the Study of Animal Behaviour.
