Carotenoids are well known for their immune-stimulant function in birds and other vertebrates. Moreover, they have potential antioxidant capacity, scavenging free radicals and protecting cell compartments from oxidation. Most essential carotenoids are fat soluble and could be stored for times of need especially in adipose tissues, built up by migratory birds as the main source of energy on long-distance flights. In an exclusive diet experiment, garden warblers (Sylvia borin) were fed ad libitum with an experimental diet, enriched with two different dose rates of carotenoids, or with control food, during the period of their first autumn migration. Plasma carotenoid content was measured via HPLC and chroma of plasma and fat examined with a spectrophotometer. Birds were infected with Isospora spp. and intensity of infection determined by oocyst counts 3 days post infection. Plasma lutein levels and chroma of subcutaneous fat stores were positively correlated and chroma values of these fat stores increased in the birds that got the higher dose rate, whereas they decreased significantly in the control group after infection with Isospora spp. Chroma of subcutaneous fat deposits in vivo and intensity of Isospora infection were negatively correlated. By measuring the chroma of fat deposits in vivo, we show that fat can be a reservoir for carotenoids. These colorful antioxidants are stored in the fat and taken from there in times of a higher demand, e.g. when mounting an immune response to parasites.