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First published online June 29, 2006
Journal of Experimental Biology 209, 2622-2627 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02293

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Metabolic substrate use and the turnover of endogenous energy reserves in broad-tailed hummingbirds (Selasphorus platycercus)

Scott A. Carleton^*, Bradley Hartman Bakken and Carlos Martínez del Rio

Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071, USA

^* Author for correspondence (e-mail: scarlet{at}uwyo.edu)

Accepted 24 April 2006

We fed broad-tailed hummingbirds (Selasphorus platycercus) diets of contrasting carbon isotope composition and measured changes in the ¹³C of expired breath through time. By measuring the ¹³C in the breath of fed and fasted birds we were able to quantify the fraction of metabolism fueled by assimilated sugars and endogenous energy reserves. These measurements also allowed us to estimate the fractional turnover of carbon in the hummingbirds' energy reserves. When hummingbirds were feeding, they fueled their metabolism largely (90%) with assimilated sugars. The rate of carbon isotope incorporation into the energy reserves of hummingbirds was higher when birds were gaining as opposed to losing body mass. The average residence time of a carbon atom in the hummingbirds' energy reserves ranged from 1 to 2 days.

Key words: ¹³C, energy storage, fuel use, hummingbird, Selasphorus platycercus, isotopic incorporation, respiration, stable isotopes

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