Fig. 3. ¹³C of expired breath in fasted and fed broad-tailed hummingbirds (Selasphorus platycercus). The dotted gray lines parallel to the x- and y-axes denote the ¹³C of diets and diet shift days, respectively. (A) After the carbon isotopic composition of the diet was changed from C₃ to C₄ (day 0 to 19), the ¹³C of exhaled CO₂ in fasted birds changed following one-compartment, first order kinetics with a fractional rate of isotopic incorporation equal to 0.47±0.19 day^-1. When the isotopic composition of the hummingbirds' diet was changed from C₄ to C₃ (day 20 to 43), the ¹³C of exhaled CO₂ in fasted birds changed with a faster fractional rate of isotopic incorporation (0.86±0.16 day^-1). Note that the asymptotic value of ¹³C in the fasted hummingbirds' breath was more negative than that of their diet (represented by dotted horizontal lines). We constructed these curves using the parameters in Table 1. (B) Immediately after a diet shift, the ¹³C of breath in fed birds changed to resemble closely (but not exactly) that of the new diet. The broken curve is the value predicted for fasted birds and is presented to compare the differences between measurements done on fed and fasted birds.