The masses of the bees used in the experiment.  Bee mass was measured after the bees had returned to the colony, offloaded their stored sucrose solution and returned to the flight arena. Bees will be minimally loaded in this situation (i.e. nearly empty crop/honey stomach) and so this should be a good measure of unloaded bee mass (see main text).  Bee “Pink 60” failed to re-emerge from the colony and so we estimated her mass using the mean mass of the other bees in the experiment.