Keeling plots for hummingbirds: a method to estimate carbon isotope ratios of respired CO₂ in small vertebrates

Abstract

The carbon isotope composition of an animal’s breath reveals the composition of the nutrients that it catabolizes for energy. Here we describe the use of Keeling plots, a method widely applied in ecosystem ecology, to measure the δ¹³C of respired CO₂ of small vertebrates. We measured the δ¹³C of Rufous Hummingbirds (Selasphorus rufus) in the laboratory and of Mourning (Zenaida macroura) and White-winged (Z. asiatica) Doves in the field. In the laboratory, when hummingbirds were fed a sucrose based C3 diet, the δ¹³C of respired CO₂ was not significantly different from that of their diet (δ¹³C_{C3 diet}). The δ¹³C of respired CO₂ for C3 fasted birds was slightly, albeit significantly, depleted in δ¹³C relative to δ¹³C_{C3 diet}. Six hours after birds were shifted to a sucrose based C4 diet, the isotopic composition of their breath revealed that birds were catabolizing a mixture of nutrients derived from both the C3 and the C4 diet. In the field, the δ¹³C of respired CO₂ from Mourning and White-winged Doves reflected that of their diets: the CAM saguaro cactus (Carnegeia gigantea) and C3 seeds, respectively. Keeling plots are an easy, effective and inexpensive method to measure δ¹³C of respired CO₂ in the lab and the field.