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Why do some bird species show dramatic sexual dichromatism in their plumage? Sexual selection is the most common answer to this question. However, other competing explanations mean it is unwise to assume that all sexual dichromatism has evolved by this mechanism. Even if sexual selection is involved, further work is necessary to determine whether dichromatism results from competition amongst rival males, or by female choice for attractive traits, or both. Here, we test whether sexually dichromatic hihi ( Notiomystis cincta) plumage is currently under sexual selection, with detailed behavioural and genetic analyses of a free-living island population. Bateman gradients measured for males and females reveal the potential for sexual selection, whilst selection gradients, relating reproductive success to specific colourful traits, show that there is stabilizing selection on white ear tuft length in males. By correlating colourful male plumage with different components of reproductive success, we show that properties of yellow plumage are most likely a product of male-male competition, whilst properties of the black and white plumage are an outcome of both male-male competition and female choice. Male plumage therefore potentially signals to multiple receivers (rival males and potential mates), and this may explain the multicoloured appearance of one of the most strikingly dichromatic species in New Zealand. [ABSTRACT FROM AUTHOR]

1. 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. 2. 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. 3. 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. 4. 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. 5. We suggest that carotenoids influence the appearance of nestling gapes both by increasing pigmentation and as a filter of UV-reflecting structures. [ABSTRACT FROM AUTHOR]