Ultra‐violet reflectance of male and female red grouse, Lagopus lagopus scoticus, sexual ornaments reflect nematode parasite intensity

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In many birds, females prefer males with the biggest or brightest sexual ornaments, which might reflect a higher phenotypic quality, such as fewer parasites. Unlike humans, most birds detect near‐ultraviolet (UV) light, and UV signals can play an important role in sexual signalling and mate choice. Using a spectrophotometer, we analysed the colour of red grouse Lagopus lagopus scoticus sexual ornaments (their combs). We first show that combs reflect both in the red (600–700 nm) and UV (300–400 nm) part of the spectrum. Second, we investigated whether comb size and colour, and UV reflectance in particular, reflected an aspect of individual quality: the intensity of infection by a main nematode parasite, the caecal threadworm Trichostrongylus tenuis. We first analysed comb size and colour variation, and parasite intensity variation, in relation to sex and age. Males had bigger and redder combs than females, but UV brightness was greater for female than for male combs. Comb colour also differed between age groups, with young birds of both sexes showing brighter UV than old birds. Young grouse also had fewer T. tenuis worms than old grouse. We further tested whether intensity of infection could be predicted from comb characteristics (size and colour) in male and female red grouse. We found that parasite intensity was not significantly related to comb size or red brightness, but fewer worms were predicted from brighter UV in combs, in both males and females. The results indicate that UV reflectance of combs have a quality revealing function and might play an important role in grouse mate choice: UV brightness of combs could enable both male and female red grouse to assess the parasite loads of a potential mate.