Shield colours of the Moorhen are differently related to bacterial presence and health parameters

Parasites have the potential to be important forces driving the evolution of honest signals (e.g. visual communication in the context of sexual selection) because they can affect the expression of colours in the host. Birds are affected by a variety of parasite taxa, but despite their acknowledged importance in epidemics, the role of bacteria in sexual selection in birds has been poorly investigated. In this study, we examined the expression of colours (red and yellow) in the frontal shield of Moorhens and compared the results with the cloacal bacterial presence. We also measured several health parameters (body condition, PHA immune response, heterophils/lymphocytes ratio, haematocrit) that could be related to the expression of frontal shield colour. We found a negative correlation between frontal shield redness and bacterial presence, but red colouration was not related to any other health index. In contrast, the colouration of the yellow portion of the bill was positively related to health (haematocrit and H/L ratio) and immune (PHA) parameters, but not to bacterial presence. These correlational data are consistent with the recent hypothesis that different kinds of colouration within an individual may provide different units of information. In the Moorhen, the frontal shields could act as a twofold honest signal of health status, where red reflects cloacal bacterial presence and yellow is related to blood parameters.