DOI 10.1007/s00442-004-1643-y METHODS

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 Humming-birds (Selasphorus rufus) in the laboratory and of Mourn-ing (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 (δ13CC3 diet). The δ 13C of respired CO2 for C3 fasted birds was slightly, albeit significantly, depleted in δ13C relative to δ13CC3 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