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 pinetorum, blooming at high altitude, was visited frequently by hummingbirds because of 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. Chilopsis linearis, blooming at lower elevations where low temperatures did not inhibit insect activity, attracted increasing numbers of bumblebees which depleted nectar levels and virtually excluded hummingbirds. The present study provides another example of competition between vertebrates 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.