Bracts and long-tube flowers of hummingbird-pollinated plants are conspicuous to hummingbirds but not to bees

Abstract

The ‘bee avoidance’ hypothesis posits that the colour of hummingbird-pollinated flowers will exclude bees, especially in long-tubed flowers, which are more subject to bee nectar robbers. However, many hummingbird-pollinated plants possess bracts, and the role of bract colour in bee avoidance has not previously been tested. We assessed the signal value of bract colours by comparing them with the petals of species with and without bracts. We expected the bracts would contrast and enhance the petal signal more to hummingbirds than to bees, especially in long-tubed flowers. We compared bract signals in the hummingbird and bee visual systems, petal signals between the bract and leaf background, and the relationship between flower length and bract colour. For species with bracts, the bracts contrasted more than petals in the hummingbird visual system, but less in the bee visual system. Petals contrasted more with the bract background than with the leaf background to hummingbirds, but not to bees. Bract colours of long-tubed flowers contrasted less than those of short-tubed flowers to bees. Our results show that bracts are attractive to hummingbirds, and have the potential to exclude antagonists in hummingbird-pollinated plants. These patterns support the bee avoidance hypothesis, especially in long-tubed flowers.