Mechanisms of Competition in Tropical Hummingbirds: Metabolic Costs for Losers and Winners

Abstract

The goal of this study was to measure directly the acquisition and allocation of energy for competing hummingbirds. In flight—cage trials providing an easily defensible food source, complete energy budgets (intake, expenditure, and storage) were obtained for two sympatric Costa Rican hummingbirds: a territorial (Amazilia saucerottei) and a low reward traplining (Chlorostilbon canivetii) species. Behavior and energy responses of solitary birds (controls) were compared to the responses of conspecific and heterospecific pairs competing for a single feeder. Relative to controls, birds in pairs generally spent more time in flight, due primarily to increased rates of visiting the feeder or to agonistic encounters, with a consequent increase in energy expenditure. Ability of birds in pairs to compensate for increased expenditures by increasing intake, thus maintaining a constant body mass over 24 h, depended upon species and dominance status. The greatest asymmetries in energy effects occurred in heterospecific pairs, in which Amazilia aggressively interfered with access to the feeder by Chlorostilbon did not increase intake and thus suffered significant loss of energy stores. Conspecific pairs of each species experienced energy effects intermediate to the extreme responses of heterospecific pairs. Individuals of either species had greater energy success when paired with a Chlorostilbon rather than with an Amazilia. For both species, individuals that were relatively more aggressive than their cagemates experienced the greater energy success, though absolute frequency of aggression of an individual was a poor indicator of its energy success. Differential energy response to competition by Amazilia and Chlorostilbon appears to be one mechanism that contributes to local species abundance and guild composition.