Spectral Mouth Colour of Nestlings Changes With Carotenoid Availability

Abstract

Despite the proliferation of studies on the role of nestling mouth colour in parent-offspring communication, there has been very little work regarding the proximate mechanism for mouth pigmentation. Carotenoids, a class of phytochemicals important for immune function and gained by birds only through their diet, also serve as pigments for yellow-orange colours. Carotenoids have been shown to be responsible for the colouration of adult plumage and integuments (e.g. bills, legs, combs of adults of several species), but their role in colouring nestling gapes, and the surrounding fleshy rictal flanges, remains hypothetical. Here, we present direct evidence for the importance of carotenoid availability in determining nestling mouth colouration. In field experiments on Hihi (Notiomystis cincta), a passerine endemic to New Zealand, we experimentally supplemented carotenoids directly to nestlings, and indirectly by provisioning parents, and measured the effect on nestling mouth colouration with spectrometer. We found that increased carotenoid availability in the diet enhanced circulating blood plasma carotenoid concentrations, and that this in turn influenced mouth palate and rictal flange colouration. High carotenoid availability increased saturation of the yellow wavelengths of the spectrum reflected by both the palate and flanges, but reduced the reflectance of ultraviolet (UV) wavelengths of the rictal flanges. We suggest that carotenoids influence the appearance of nestling gapes both by increasing pigmentation and as a filter of UV-reflecting structures.