Elevation and the Morphology, Flight Energetics, and Foraging Ecology of Tropical Hummingbirds

Abstract

The power that a hummingbird must expend to hover increases with decreasing air density and therefore with increasing elevation. Equations are available for estimating this power output requirement and for computing the easily obtained parameter of wing disc loading (ratio of body weight to area of disc whose diameter is wing span), to which power output is directly related. We test four related predictions based on these equations and on empirical patterns of exploitation versus interference competition among hummingbirds. (1) Species having the same morphology over broad elevational range are expected to engage in more interference competition at high elevations. (2) The allowable extremes of power output for hovering should be relatively constant with elevation. (3) Mean wing disc loading of hummingbird assemblages should decrease with increasing elevation. (4) The mean power output among hummingbird assemblages should not vary with elevation. Tests with field data verify the last three predictions; although independent data for testing the first prediction directly are not yet available, its validity is confirmed by a clearcut corollary. These results imply a precise interrelationship between hummingbird behavior, morphology, energetics, and competition.