The nectars of two species of shrubs, Ribes pinetorum and Chilopsis linearis, were limited food resources that were utilized by both hummingbirds and insects. Both shrub species had bimodal patterns of daily nectar secretion with peaks in morning and afternoon. Ribes pinet- orum, blooming at high altitude, was visited frequently by hummingbirds because its timing of nectar secretion coincided with periods of low temperature when insects were inactive. Sufficient nectar accumulated in flowers to make hummingbird foraging economical in early morning and late afternoon, but not at midday when insects were foraging and had lowered nectar levels. Chi- lopsis linearis, blooming at lower elevations where low temperatures did not inhibit insect activ- ity, attracted increasing numbers of bumblebees which depleted nectar levels and virtually excluded hummingbirds. The present study provides another example of competition between ver- tebrates and insects, and suggests that this interaction between distantly related taxa may play a significant role in plant-pollinator coevolution as well as in community ecology. Although many species of flowering plants have coevolved highly specific relationships with animal pollinators (Faegri and van der Pijl, 1971; Grant and Grant, 1965, 1968; Proctor and Yeo, 1972; van der Pijl and Dodson, 1967). Others are relative generalists. They may be pollinated by several animal taxa which forage for their nectar or pollen. Animals may compete for these food resources. Competition for floral rewards between individuals of closely related species is well documented and is thought to play a major role in the organization of guilds of nectar-feeding animals (e.g., Feinsinger, 1976; Heinrich, 1976; Inouye, 1978; Gill, 1978; Gill and Wolf, 1975; Kodric- Brown and Brown, 1978; and many others). Several recent studies have sug- gested that organisms in different classes and phyla compete for food and other resources (e.g., Brown and Davidson, 1977; Reichman, 1979). Since two classes of vertebrates (birds and mammals) and several orders of insects (Hymenoptera, Lepidoptera, Diptera, and Coleoptera) commonly feed on nectar or pollen and serve as pollinators, competition between these dis- tantly related taxa may play a significant role in the ecological relationships among these flower visitors and in the coevolution of plants and their polli- nators (Carpenter, 1979; Kodric-Brown and Brown, 1979). In the present paper we assess competition between hummingbirds and bees for the nectar of two Southwestern shrubs, Ribes pinetorum Greene (Saxifragaceae), orange gooseberry, and Chilopsis linearis (Cav.) Sweet, (Bignoniaceae), desert willow.