Many animal species have bright, carotenoid-based integumentary coloration that is an important criterion in female choice. It has generally been assumed that carotenoid-based colour displays act as signals of quality because they reflect the foraging ability of the bearer, but this hypothesis has not been tested. In birds, carotenoid pigmentation of feathers is deposited at the time of moult. During moult, feathers grow in regular daily cycles resulting in `growth bars' that provide a record of the rate of feather growth. To test the idea that the brightness of carotenoid coloration reflects nutritional condition during moult, we compared the brightness of carotenoid-based plumage coloration with both feather growth rate and timing of moult in male house finches (Carpodacus mexicanus). Among four populations with substantial differences in mean male plumage brightness, there was no significant variation in mean feather growth rate. Thus, the reduced brightness of some populations is not the result of reduced access to food per se. Within populations, we found a significant positive relation between the growth rate of a male's tail feathers and the brightness of his plumage, suggesting that males growing brighter feathers are in better nutritional condition. The growth rate of tail feathers of captive males provided with ad libitum food was also significantly greater than the growth rate of males in any wild populations. Within populations, we also found a significant negative relation between the onset of moult and plumage colour, with males growing brighter feathers starting moulting earlier. These observations provide support for the hypothesis that carotenoid-based plumage coloration is an indicator of nutritional condition during moult. Variation in nutritional condition may arise from differences among individuals in either their foraging ability or their health.