THE IMPACT OF VARIATION IN STOPOVER HABITAT QUALITY ON MIGRANT RUFOUSHUMMINGBIRDS

We examined the impact of variation in habitat quality on migrating Ru fous Hummingbirds (Selasphorus rufus) in the California Sierra Nevada As do other migratory species, these birds depend on ''stopover'' habi tats en route for feeding and replenishing depleted energy stores. Dur ing seven years of study, the quality of the stopover habitat (assesse d in terms of the density of nectar food resources) varied widely due to natural variation in flowering. In years when stopover habitat qual ity was poor incoming body masses were low and stopover durations were long. Population densities of migrant hummingbirds at the study site were coupled to habitat quality both within and among years. These obs ervations demonstrate important effects of stopover habitat variation on the physiological, behavioral, and population ecology of migrating hummingbirds. High-quality stopover habitats are critical links betwee n breeding and wintering areas for many species, and their preservatio n should be considered an essential component of strategies aiming to conserve migratory bird populations.