Of hummingbirds and helicopters: Hovering costs, competitive ability, and foraging strategies

Abstract

Wing morphology and flight kinematics profoundly influence foraging costs and the overall behavioral ecology of hummingbirds. By analogy with helicopters, previous energetic studies have applied the momentum theory of aircraft propellers to estimate hovering costs from wing disc loading (WDL), a parameter incorporating wingspan (or length) and body mass. Variation in WDL has been used to elucidate differences either among hummingbird species in nectar-foraging strategies (e.g., territoriality, traplining) and dominance relations or among gender-age categories within species. We first demonstrate that WDL, as typically calculated, is an unreliable predictor of hovering (induced power) costs; predictive power is increased when calculations use wing length instead of wingspan and when actual wing stroke amplitudes are incorporated. We next evaluate the hypotheses that foraging strategy and competitive ability are functions of WDL, using our data in combination with those of published sources. Variation in hummingbird behavior cannot be easily classified using WDL and instead is correlated with a diversity of morphological and physiological traits. Evaluating selection pressures on hummingbird wings will require moving beyond wing and body mass measurements to include the assessment of the aerodynamic forces, power requirements, and power reserves of hovering, forward flight, and maneuvering. However, the WDLhelicopter dynamics model has been instrumental in calling attention to the importance of comparative wing morphology and related aerodynamics for understanding the behavioral ecology of hummingbirds.

Additional Information

© 2004 by The University of Chicago. Submitted October 9, 2002; accepted July 9, 2003. Electronically published January 28, 2004. We thank an anonymous reviewer for comments on the manuscript. For assistance with fieldwork in Colorado, we thank the Colorado Springs School, P. Baik, C. Evans, A. Gilbert, B. Holmes-Stanciu, O. Starry, and D. Stephens. For assistance with fieldwork in Peru, we thank C. Barber, R. Gibbons, W. Palomino, M. Van Vlaardingen, and R. Yabar. We also gratefully acknowledge the Earthwatch volunteers who made the trip to Peru at their own expense to assist with our research. Funding was provided by grants from the National Science Foundation (IBN-9601089, IBN-9817138, IBN-992155), the Earthwatch Institute, Sigma Xi, and the University of Texas at Austin.

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Supplemental Material - ALTamnat04table2.txt

Supplemental Material - ALTamnat04tableA1.txt

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