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

Abstract

SUMMARY We fed broad-tailed hummingbirds (Selasphorus platycercus) diets of contrasting carbon isotope composition and measured changes in theδ 13C of expired breath through time. By measuring theδ 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.

Stable Isotope Research in Southern African Birds

“…Tissues that are metabolically active will reflect the average diet of the individual during a certain period into the past, according to the tissue's turnover rate, ranging from days (liver and blood plasma), to weeks (muscle and whole blood), to months, years or a lifetime (bone collagen) (Hobson and Clark, 1992a;Hobson, 1999;Hobson, 2005;Podlesak et al, 2005;Inger and Bearhop, 2008;Larson and Hobson, 2009;Hobson, 2011). Breath, reflecting immediately metabolised nutrients, and faeces representing the isotopic signature of a recent meal, may also be sampled non-destructively (Podlesak et al, 2005;Carleton et al, 2006;Voigt et al, 2008;. Therefore, by analysing different tissues with different turnover rates temporal and/or spatial trophic shifts may be assessed.…”

“…Recent works therefore suggest that caution be applied in the use of isotope models because of uncertainty in diet-tissue fractionation factors for a wide variety of organisms and conditions (Caut et al, 2008b). Some have considered nectar feeding species and breath samples of broad-tailed hummingbirds Selaphorus platycercus, in a study that controlled diet, the diet-breath fractionation factor was calculated at -2.3‰ to -1.6‰ (Carleton et al, 2006).…”

Oxidation rate and turnover of ingested sugar in hovering Anna's(Calypte anna) and rufous (Selasphorus rufus)hummingbirds

“…Thus, instantaneous estimates of ␦ 13 C breath values were possible. We assume that the incorporation of carbon into expired CO 2 can be approximated by single-compartment, firstorder kinetics (Carleton et al, 2006). The non-linear fitting formula is: …”

“…Alternatively, ingested sugars may remain in the pool of actively metabolized substrates for a brief period of time, being rapidly replaced by newly ingested sugar molecules as foraging proceeds. Carleton et al (Carleton et al, 2006) previously estimated the turnover rate of carbon in endogenous stores of energy (fat) over a period of several days, but this work was not designed to examine carbon turnover at the shorter time scales relevant to birds engaged in repeated foraging bouts. The study presented here expands on these previous studies and quantifies the turnover rate of ingested sugar in the pool of actively metabolized substrates in actively foraging rufous (Selasphorus rufus) and Anna's (Calypte anna) hummingbirds.…”

“…The methodology employed in this and previous studies (Carleton et al, 2006;Carleton et al, 2004;Welch et al, 2006) takes advantage of the difference in stable isotopic signature of carbon in sugar derived from two distinct plant sources. Sugar from sugar beets displays a carbon stable isotope ratio particular to plants with Hummingbirds obtain most of their dietary calories from floral nectar ingested during hovering flight.…”