Visit larger displays but probe proportionally fewer flowers: Counterintuitive behaviour of nectar-collecting bumble bees achieves an ideal free distribution

1. Patterns of pollinator responses to variation in floral display size have significance for pollen flow among plants. Here we test a theoretical model for explaining such patterns by simultaneously assessing bumble bee behaviour and nectar availability in two native stands of Cirsium purpuratum with different spatial densities. 2. A bumble bee (Bombus diversus) foraging on a plant remembered and avoided only one or two flower heads that it had probed before, so that the flower-head revisitation rate increased as it stayed longer on a plant. Moreover, the revisitation rate increased less rapidly on larger displays. 3. The number of heads probed per plant increased less than proportionally with display size, and this increase was smaller at higher plant density. This pattern is consistent with our expectation that a bee leaves a plant when the cost of flower-head revisitation exceeds that of interplant movement. However, bees left plants slightly earlier than predicted. 4. As predicted, the visitation rate of bees per plant showed a decelerated increase with floral display size, and this increase was greater at higher plant density. 5. As a result of these complementary changes in the number of heads probed per plant and visitation rate per plant across plant densities, nectar rewards per head were equalized among displays (an ideal free distribution was achieved).