A Female-Specific Color Signal? Black-Mottled Bills Indicate Breeding in Female Common Waxbills

AbstractFemale ornamentation is common in birds but usually resembles that of males. In contrast to this general pattern, here we show that the red bill of wild adult common waxbills (Estrilda astrild) often becomes mottled with black when females breed. This color change is not explained by reallocation of red carotenoid pigments away from the bill but requires deposition of melanin pigments. The change is very noticeable and makes female bills resemble the black bill of nestlings and fledglings. Perhaps this color change exploits useful innate responses of males toward nestlings, such as ceasing mating-related behavior and initiating parental care. Unlike the vast majority of female signals and ornaments, black-mottled bills are not derived from a male trait, but they are derived from a nestling trait, in accordance with the idea that color signals often evolve using preexisting developmental paths.