Brighter is better: bill fluorescence increases social attraction in a colonial seabird and reveals a potential link with foraging

Crested auklets ( Aethia cristatella ) are colonial seabirds with brilliant orange bills during the breeding season. We characterized the bill pigment with spectroscopy methods (resonance Raman, fluorescence, absorbance). We excluded carotenoids as a possible chromophore and showed that the pigment most closely resembles pterins. Like pterins, the bill pigment fluoresces, and it occurred in two phenotypes that may differ geographically, perhaps due to environmental heterogeneity. The pigment is unique in the genus Aethia , and its spectra did not match any known molecule. The UV–Vis absorbance spectrum of the bill pigment overlaps with the extracted pigment of euphausiids, a favored food of crested auklets. A color preference associated with prey may have favored characteristics of the crested auklet’s accessory bill plates. Crest size, a signal of dominance, tended to correlate positively with the highest fluorescence in the single-band phenotype. Brighter bills may function in self-advertisement and verify the status signal of the crest ornament. We tested for a behavioral preference using identical decoys that differed only in bill fluorescence. Crested auklets approached models with fluorescent bills at a higher frequency. In cases where the sex of crested auklets was known, males responded at a higher frequency to fluorescent bills, but females did not. In an evolutionary context, bill fluorescence could have conferred an advantage in social interactions, e.g., in dimly lit rock crevices. Bill brightness and color may communicate success in foraging and may function as an honest signal of mate quality. Significance statement Preferences embedded in sensory systems may influence the evolution of ornamental traits. Overlap in chemical properties of zooplankton and brilliant orange bills suggests that crested auklets prefer a bill pigment patterned on a food preference. The pigment is unique in its genus, but occurs in two fluorescent phenotypes that may vary geographically, possibly due to environmental heterogeneity. The pigment resembles a class of compounds known as pterins but could be novel. Rictal plates are bill plates that flare out from the corners of the mouth, and they tend to be brighter in crested auklets with larger feather crests. Birds with larger crests are more attractive and dominant. Brighter bills may advertise foraging success and verify status. Bill fluorescence is attractive to crested auklets. Crested auklets approached decoys w