The health signalling of ornamental traits in the Grey Partridge (Perdix perdix)

Abstract

Birds express various secondary ornaments that can indicate individual condition and health. Amongst these, red-coloured carotenoid-based ornaments are supposed to be particularly valuable predictors of individual quality, due to their sensitivity to oxidative stress. Nevertheless, melanin-pigmented traits might also signal health and immune functions. Both types of ornaments may be either skin-based or feather-based, each differing in their dynamics. In the present study, we compared the health- and stress-indicating capacity of melanin-based feather ornamentation and putatively carotenoid-based skin ornamentation in a single species—the Grey Partridge (Perdix perdix), a vulnerable avian species of the European agricultural landscape. In captive males, we firstly verified the carotenoid content of the red-coloured skin tissue behind the eye by chromatography (HPLC). Secondly, we assessed the individual health status of all males by examining differential leukocyte count, the frequency of immature erythrocytes, malaria prevalence and proinflammatory immune responsiveness to phytohaemagglutinin (PHA). Both the size of the melanin-based ornament and red chroma of the carotenoid-based ornament were related to the heterophil:lymphocyte (H/L) ratio. Hence, in the Grey Partridge, both redness of the skin ornament and area of the feather ornament may serve as honest indicators of individual health and long-term stress. However, the two ornamental components were unrelated to each other, and the directions of their association to the H/L ratio were opposite. We therefore propose that, in this species, larger melanin-based feather ornamentation size is linked to higher levels of stress (possibly caused by more intensive social interactions with other males), while the level of expression of the carotenoid-based skin ornamentation more reliably signals actual individual health status. Our results are potentially valuable from the perspective of Grey Partridge conservation efforts, as they indicate a simple method for assessing individual quality in this species.