Carbon isotope ratios in exhaled CO₂ can be used to determine not just present, but also past diets in birds

We show that an animal's past and present diet can be distinguished through the δ¹³C of exhaled CO₂. The exhaled δ¹³C of 12 pigeons fed solely corn (a C₄ plant) for 30 days was -13.63‰ (±0.30). We then fed six pigeons wheat (a C₃ plant) and continued to feed the other six corn. After 48 h the exhaled δ¹³C from the corn-fed pigeons was unchanged; that from the wheat-fed pigeons was -20.5‰. We then fasted three of the wheat-fed pigeons for 3 days, after which their exhaled δ¹³C was -14.96‰, while it was -13.57‰ in corn-fed pigeons, and -22.22‰ in pigeons that continued on wheat. Thus, we could infer diet from the ¹³C/¹²C ratios of exhaled CO₂. Significantly, breath samples from fasted pigeons also revealed that they had eaten corn when their lipid stores were formed. We also showed that the change in the ¹³C/¹²C of exhaled CO₂ had a half-life of approximately 3.5 h, and a time constant of approximately 6.7 h. Thus one can infer past and present diet from exhaled δ¹³C alone, if the initial breath sample is followed by a fasted breath sample, without harming the animal or having to recapture it successively.