Shield colours of the Moorhen are differently related to bacterial presence and health parameters S. FENOGLIO 1,3 , M. CUCCO 1 , L. FRACCHIA 2 , M.G. MARTINOTTI 2 and G. MALACARNE 1 1 Università del Piemonte Orientale, Dipartimento di Scienze Chimiche, Alimentari, Farmaceutiche e Farmacologiche, Via Cavour 84, I-15100, Alessandria, Italy 2 Università del Piemonte Orientale, Dipartimento di Scienze dell’Ambiente e della Vita, Via Bovio 6, I-28100 Novara, Italy Received 7 January 2004, accepted 25 February 2004 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 epi- demics, the role of bacteria in sexual selection in birds has been poorly investi- gated. 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 bacter- ial 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 correla- tion between frontal shield redness and bacterial presence, but red colouration was not related to any other health index. In contrast, the colouration of the yel- low portion of the bill was positively related to health (haematocrit and H/L ratio) and immune (PHA) parameters, but not to bacterial presence. These corre- lational 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. KEY WORDS: bacteria, colour expression, honest signal, immune system, Moorhen, parasites. Introduction . . . . . . . . . . . . . . . . . 172 Methods . . . . . . . . . . . . . . . . . . 172 Results . . . . . . . . . . . . . . . . . . 174 Discussion . . . . . . . . . . . . . . . . . 176 Acknowledgements . . . . . . . . . . . . . . . 177 References . . . . . . . . . . . . . . . . . 177 Appendix 1 . . . . . . . . . . . . . . . . . 180 Ethology Ecology & Evolution 16: 171-180, 2004 3 Corresponding author: Stefano Fenoglio (E-mail: fenoglio@unipmn.it; Fax: +39 0131 283841).