WELCH, KC*; BAKKEN, BH; SUAREZ, RK; Univ. of California, Santa Barbara; Univ. of Wyoming; Univ. of California, Santa Barbara: Use of Dietary Sugar by Hummingbirds as Revealed by Stable Isotope and Respiratory Analysis

Hummingbirds are noted for their high foraging frequencies, their primarily nectar-based (sugar) diet, and their capacity to routinely display RQ values near 1.0. These observations led Suarez, et al. (1990, Proc. Natl. Acad. Sci., USA 87: 9207-9210) to propose that hummingbirds are able to fuel most or all of their metabolism during foraging with recently ingested sugars. We tested the hypothesis that broad-tailed hummingbirds (Selasphorus platycercus) rely upon direct oxidation of diet-derived sucrose to fuel flight when allowed free access to sugar water. Hummingbirds were maintained on a diet containing beet sugar, leading to carbohydrate and fat stores with ¹³C/¹²C isotope ratios characteristic of this C3 plants. Following a fast, hummingbirds were then given access to a feeder containing a solution containing cane sugar, i.e., from a C4 plant source. The feeders were fitted with a mask to allow flow through respirometry. By subsampling and analyzing the ¹³C/¹²C ratio of CO₂ expired while birds fed and comparing these values with RQ values, we determined that the hummingbirds began oxidizing sucrose from the C4 plant source within a few minutes after their first meal and could support most of their hovering metabolism with recently ingested sugar. This is the first report of a vertebrate fueling high metabolic rates during exercise mainly with dietary sugar.