Floral display size influences subsequent plant choice by bumble bees

1. The effect of floral display size (number of open flowers per plant) on subsequent plant choice by bumble bees, Bombus hypocrita sapporensis (Cockerell), was investigated using artificial inflorescences with blue or yellow flowers representing different 'species'. 2. Bees flew to an inflorescence of the same colour (constant flight) more often after visiting a large inflorescence. This response occurred when the nectar volume per flower was constant (larger inflorescence offered more nectar), and when nectar volume per inflorescence was constant (nectar volume per flower varied with display size). These results indicate that bees did not exhibit higher constancy after visiting larger inflorescences because they extracted more nectar than from smaller ones. 3. Bees spent more time during visits to large vs small inflorescences. Possibly information about the last-visited inflorescence fades from short-term memory during longer visits, which strengthens colour constancy. 4. Selection on floral display size is thought to balance the attraction benefits of large displays with the cost of geitonogamy. However, in habitats with several competing plant species, selection for colour constancy may provide a further reason for selection to favour large floral displays.