Organization of a Tropical Guild of Nectarivorous Birds

Abstract

Fourteen months' observations on hummingbird foraging patterns in successional habitats at Monteverde, Costa Rica, showed that one territorial species (Amazilia saucerottei) dominated rich resource clumps, modified all other species' patterns, and thus organized the nectarivorous bird guild. The principal nonterritorial species (Chlorostilbon canivetii), which Amazilia usually excluded from rich resources, traplined dispersed flowers and interfered with foraging patterns of other nonterritorial species. The 12 additional hummingbird species that foraged in the study habitats included species important in nearby communities, specialists on particular resources, and highly migratory opportunists. Foraging pattern diverged along several dimensions, including (1) the species or flower density of the individual plant; (2) the strata of flowers within the plant; and, since nectar was renewed at variable rates, (3) the time of day. Analysis of foraging patterns along these dimensions required that a hierarchy of niche breadth and overlap measures be defined and contrasted. Patterns compared over an entire year showed that the two principal species exploited the broadest niches, but overlapped only 17%. In no case did overall foraging patterns overlap more than 21%, and overlaps between many approached zero. These values, however, did not reveal the degree of exclusion from resources that were potentially exploitable, or the intensity of competition. Statistical correlations showed that most month—to—month changes in niche breadth, niche overlap, and population size could be attributed to shifts in the resource base. Of the 16 plant species exploited by hummingbirds, the forb Lobelia laxiflora and the tree Inga brenesii were responsible for most fluctuations in resource levels. Flowering peaks of both species attracted large numbers of hummingbirds; in general, numbers and diversity of hummingbirds rose with increased flower abundance rather than increased flower diversity. Lobelia nectar was effectively superabundant, and niche overlap during Lobelia peaks was high. Birds foraging in Inga trees, however, could diverge along spatial and diurnal dimensions, and overlap during Inga peaks was much lower. In between Lobelia and Inga peaks, the two principal hummingbird species usually dominated the guild, and expanded their niches but overlapped little. Each hummingbird species responded to that particular patterns of resource states it could best exploit, and the guild as a whole tracked the entire resource base. The cycle of resource flushes entrained these patterns to an annual rhythm.