Annual, sexual, size- and condition-related variation in the colour and fluorescent pigment content of yellow crest-feathers in Snares Penguins (Eudyptes robustus)

Colourful plumages observed in many birds provide ideal systems for investigating the control and function of animal signals, because we can identify the molecular currency (e.g. pigments, tissue structures) and thus the challenges associated with the production of the trait. To date, most attention has been paid to the signalling costs and benefits of the common pigmentary (e.g. carotenoid- and melanin-based) plumage colours. However, in penguins, the colour of feathers of crests or patches of head plumage is based on an unusual fluorescent pigment, presently thought to be in the class of pterin pigments, which are typically found as colourants in other animals like butterflies and amphibians. These yellow-orange colours may be used in sexual or social communication, but to date little is known about the signalling potential of such ornaments. We collected yellow crest-feathers from adult male and female Snares Penguins (Eudyptes robustus) in two years (2002 and 2003) and examined differences in colour and pigment content as a function of year, sex, body size and body condition. Unlike the typical pattern of birds, crests of female Snares Penguins contained higher concentrations of pigments than those of males. In both years, yellower feathers in females contained more yellow pigment. Birds of both sexes grew yellower and more pigment-rich feathers in 2002—a year in which birds were in better condition—than in 2003. Males with yellower crests also tended to be larger, heavier and in better condition, regardless of year. These results highlight the potential for yellow crests in Snares Penguins to serve as a condition-dependent indicator to conspecifics. Further work is needed to determine whether or not this plumage trait is a signal of social status or attractiveness, as colourful plumage patches appear to be in other penguin species.