Organization of a Tropical Island Community of Hummingbirds and Flowers

We studied the temporal and spatial relationships among hummingbirds and hummingbird—pollinated flowers on southwestern Puretor Rico in order to assess the roles of competition and mutualism in the organization of this community. We recorded flowering phenology, nectar production, hummingbird and nectar—robber visits to flowers, and pollen loads, and made morphological measurements of both hummingbirds and flowers along an elevational transect during two 6—mo periods. The community consists of three species of hummingbirds and at least 13 species of plants with tubular flowers specialized for hummingbird pollination. Two large hummingbirds, Anthracothorax dominicus and A. virdis, occupy largely nonoverlapping foraging areas, habitats, and elevations but overlap in their utilization of eight species of flowers that have long tubes and secrete copious nectar. The other hummingbird, Chlorostilbon maugaeus, is much smaller and forages legitimately almost exclusively on the other five species of flowers which have shorter tubes and secreteless nectar. Thus the community consists of two largely nonoverlapping subunits. Specificity is maintained because long—tubed flowers restrict legitimate foraging to large, long—billed hummingbirds, which are attracted to their high nectar levels, and because the short—tubed flowers secrete sufficient nectar for economical foraging by small, but not large hummingbirds. Both competitive and mutualistic interactions appear to have been important in the coevolution of these two discrete subcommunities of hummingbirds and flowers.