Carbon isotope ratios in exhaled CO(2) 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 delta(13)C of exhaled CO(2). The exhaled delta(13)C of 12 pigeons fed solely corn (a C(4) plant) for 30 days was -13.63 per thousand (+/-0.30). We then fed six pigeons wheat (a C(3) plant) and continued to feed the other six corn. After 48 h the exhaled delta(13)C from the corn-fed pigeons was unchanged; that from the wheat-fed pigeons was -20.5 per thousand. We then fasted three of the wheat-fed pigeons for 3 days, after which their exhaled delta(13)C was -14.96 per thousand, while it was -13.57 per thousand in corn-fed pigeons, and -22.22 per thousand in pigeons that continued on wheat. Thus, we could infer diet from the (13)C/(12)C ratios of exhaled CO(2). 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 (13)C/(12)C of exhaled CO(2) 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 delta(13)C alone, if the initial breath sample is followed by a fasted breath sample, without harming the animal or having to recapture it successively.