Altricial nestlings compete with their nest mates for resources delivered by parents. Parents may allocate food to nestlings based on reproductive value of offspring. To test the hypothesis that mouth coloration acts as a signal of nestling condition in the barn swallow Hirundo rustica, we investigated whether gape coloration is correlated with offspring quality and age. We also examined the role of ultraviolet (UV) flange coloration in parental allocation in a manipulative experiment. Mouth coloration changed with age, probably due to accumulation of dietary carotenoids in the tissue and an increase in the number of collagen layers. Highly UV and redder palates and brighter flanges were associated with longer tarsi and greater body mass at day 6 and with feather growth at day 12 posthatching. Although we did not find evidence that UV coloration of flanges is associated with nestling quality, parents preferentially fed young whose flanges reflected higher UV light, compared with experimentally UV-filtered nestlings. These results support the hypothesis that mouth coloration is a reliable signal of nestling condition. In addition, they show that UV flange coloration influences parental decisions regarding food allocation. Copyright 2007, Oxford University Press.