Carotenoids are finite resources that animals can allocate to selfmaintenance, attractiveness or reproduction. Here we test how carotenoids affect the acute phase response (APR), an intense rapid systemic response characterized by fever, sickness behavior and production of acute phase proteins, which serves to reduce pathogen persistence. We conducted a 2×2 factorial design experiment in captive adult male and female zebra finches (Taeniopygia guttata) to determine the effects of carotenoid supplementation on the intensity of the APR. We measured changes in feeding rate, activity level and body temperature of the birds. We found that, relative to unsupplemented controls, carotenoidsupplemented birds exhibited less severe reductions in feeding and activity, smaller increases in body temperature and lower circulating levels of haptoglobin (an acute phase protein) 24 h after inducing an APR. Among supplemented individuals, those with higher blood carotenoid levels exhibited a lower reduction in activity rate after 24 h. Forty-eight hours after APR induction, birds exhibited a significant decrease in plasma carotenoid levels and a decrease in bill hue, with less reduction in hue in carotenoid-supplemented individuals. These results demonstrate that carotenoids can alleviate several important behavioral and physiological effects of an APR and that bill color can change rapidly following induction of the costly APR immune defense. In particular, immune activation may have caused birds to preferentially draw down carotenoids from the bloodstream, ostensibly for use in health. Rapid bill color changes over a 48-h period support growing evidence that bills may serve as short-term signals of health and condition.