Hummingbird Behavior and Mechanisms of Selection on Flower Color in Ipomopsis

Abstract

Apparent associations between flower color and type of pollinator might be explained by correlations between color and other floral traits, rather than by narrow pollinator preferences for color. To explore this possibility, we obtained flowers varying naturally in color, nectar reward, and morphology, from a hybrid zone between Ipomopsis aggregata and I. tenuituba in western Colorado, United States. We used aviary and field experiments to study preferences of hummingbirds for unmanipulated flowers and for flowers in which we experimentally dissociated color from nectar reward and morphology. Hummingbirds preferred red flowers of I. aggregata, which contain superior nectar rewards and have relatively broad corolla tubes, relative to flowers of I. tenuituba, which are white, contain small nectar rewards, and have narrow tubes. There was no evidence of flower constancy. When presented with flowers differing only in color, birds showed a spontaneous preference for red. However, this preference could be reversed by making white flowers more rewarding than red. When plants of both parental species and their hybrids were placed in an array in the field, with all flowers painted red, hummingbirds preferred to visit the more rewarding, wider tubed I. aggregata. These results suggest that pollinator-mediated selection could act on color, shape, and nectar separately. Experimental manipulations that dissociate these traits are essential to distinguish direct from correlated selection. Also, hummingbird selection favoring red flowers may result from past experience and, thus, may depend on the ecological context.