Does resource competition among pollinators explain the evolution of red flowers in Dudleya cymosa?

Models for the evolution of floral diversity based on optimal foraging theory suggest that resource competition between floral visitors will frequently result in character displacement in floral phenotype.  In particular, theory predicts that red flowers may evolve if they are relatively inconspicuous to bees but not birds, and if birds and bees compete for nectar.  I measured pollinator response to natural color variation in the flowers of Dudleya cymosa (which range from yellow to red) in populations that differed in relative visitation by different bee species and by hummingbirds.  Hummingbirds showed no color preference, even though nectar-foraging bees were common in all populations.  Most bees showed strong preferences for yellow flowers, whether or not hummingbirds were present, but two bee species showed no color preference.  The solitary bee, Anthophora californica, which is one of the most common pollinators of D. cymosa, preferred red flowers in the presence of a congener, A. phaceliae, that had an extremely strong preference for yellow flowers, but preferred yellow flowers otherwise.  These results suggest that resource competition between birds and bees may not contribute to the evolution of red flowers in D. cymosa, and that, contrary to expectation, flower color may be used as a cue for competing bees to partition floral resources.