Wing Disc Loading: Implications and Importance for Hummingbird Energetics

Abstract

Epting and Casey (1973) pointed out that weight-specific power output (power/g) for hovering flight is correlated with wing disc loading (body weight divided by the disc area covered by the wings). Weight-specific power output for hovering flight was previously shown to be independent of body size (Hainsworth and Wolf 1972). The importance of size-related variations in energy expenditures per unit time as they might influence the ability of hummingbirds to exploit food resources was examined. Differences in energy expenditures per unit time might influence foraging efficiency and, consequently, foraging strategies, habitat occupancy, flower pollination and the evolution of body size. The equation used manipulates 2 variables, wing length and body weight. Because wing disc loading varies among species, longer wings relative to body size should decrease energy expenditure for hovering. Larger hummingbirds may have evolved longer wings relative to their weight than smaller hummingbirds to counter the effects on energy expenditure of their larger size and to increase their efficiency in foraging. It is predicted that wing disc loading should decrease with increasing body size. For the 40 spp. under consideration, there is a significant negative correlation between wing disc loading and body size (r = -.38, P < .05), suggesting that hummingbirds have compensated somewhat for increased costs of hovering associated with size by making adjustments in wing length. Hummingbirds as a group have evolved longer wings relative to their size than nonhovering species of birds (Greenwalt 1962), and this is probably related to the high energetic demands associated with hovering as a general foraging strategy. The explanation of variations in wing disc loading for individual hummingbirds involves some estimate of the magnitude of change in power input expected from variation in wing disc loading, assuming that the ratio of power input to power output is constant. Epting and Casey (1973) point out that their calculated extremes in wing disc loading would generate a difference in power input of 100 cal/g per h, assuming respiratory quotient [RQ] = 1.0 and efficiency = 5.5% (Hainsworth and Wolf). Differences between other species will be less, and this is perhaps reflected in the lack of difference in empirical measures of power input for hummingbirds that differ somewhat in wing disc loading. The absolute magnitude of the effect of variation in wing disc loading depends on body size. Epting and Casey have introduced an interesting correlation with important implications for comparisons of hummingbird energetics, particularly for large species. Further empirical evidence on O2 consumption of hummingbirds with different wing disc loadings is not necessary to draw such implications for the ecology and evolution of hummingbirds. Differences in efficiency of energy transfer from power input to power output represent only 1 factor that could influence foraging energetics and foraging efficiency.