Mouth coloration in nestling Cave Swallows (Petrochelidon fulva) differs from that of adults, is carotenoid based and correlated with body mass

The mouth coloration of nestling birds is hypothesized to be uniquely adaptive during the nestling stage, perhaps signalling condition. To identify nestling-specific colours, we compared adult and nestling gapes in Cave Swallows (Petrochelidon fulva). Nestling gapes reflected more ultraviolet (UV) light and tended to be yellower (because they contained carotenoids). Across broods, heavier nestlings had yellower, less bright, and less UV-rich flanges, and less bright gapes. Nestlings with carotenoid-rich plasma had brighter flanges and yellower gapes. Color did not, however, signal within-brood status. Studies of nestling mouth coloration may benefit from considering complementary and/or competing roles of the trait across ontogeny.