Effect of Flower Shape and Size on Foraging Performance and Trade-Offs in a Tropical Hummingbird

Matches between the bills of hummingbirds and the flowers they visit have been interpreted as examples of coadaptation and feeding specialization. Observations of birds feeding at flowers longer or shorter than their bills combined with a lack of experimental evidence for foraging trade-offs, however, fail to support these interpretations. We addressed these inconsistencies by considering a seldom-studied dimension of hummingbird-flower relationships, the shape of bills and flowers, through experiments on the Purple-throated Carib, Eulampis jugularis, and its major food plant, Heliconia, in the eastern Caribbean. Bills of male E. jugularis are considerably shorter and straighter than bills of females. We examined foraging performances and trade-offs during visits to natural heliconias and 34 artificial flowers of differing length and curvature. Supporting predictions based on matches between bill and flower morphology, handling times of females were significantly shorter than those of males at the long, curved flowers of a green morph of H. bihai. Contrary to predictions, handling times of males were not significantly shorter than handling times of females at the short flowers of H. caribaea. At artificial flowers, maximum extraction depths of females were significantly longer than maximum extraction depths of males at all curved flowers, but not at straight flowers. Handling times of females were significantly shorter than handling times of males at the longest artificial flowers for all curvatures, whereas handling times of males were significantly shorter at short, straight, artificial flowers, but only while hover-feeding without a perch. Within each sex, handling times were inversely related to bill length at long flowers for all shapes. Taken together, these performance trade-offs suggest that the long, curved bills of females are adapted for feeding from long, curved flowers, whereas the short bills of males are adapted for hover-feeding from short, straighter flowers. In addition, the finding that differences in feeding performance occur at the extremes of floral phenotypes suggests that the evolution of bill morphology may be driven by a small subset of the flowers visited by a hummingbird species.