ORIGINAL ARTICLE Matteo Griggio Æ Valeria Zanollo Æ Herbert Hoi UV plumage color is an honest signal of quality in male budgerigars Received: 10 January 2009 / Accepted: 15 June 2009 Ó The Ecological Society of Japan 2009 Abstract Elaborate and colorful feathers are important traits in female mate choice in birds. Plumage coloration can result from pigments deposited in feathers such as carotenoids and melanins, or can be caused by nano- scale reflective tissues (structurally based coloration), usually producing ultraviolet (UV) coloration. Struc- tural colorations remain the least studied of the three most important feather colorations. Previous studies have found a female preference for UV color in the budgerigar, Melopsittacus undulatus, but it is not clear what information this ornament conveys, nor what is the possible cost associated with its production. We inves- tigated possible correlations between immune response and plumage color of wild-type (green) male budgeri- gars. In particular we measured the wing web swelling resulting from injection of phytohaemagglutinin (PHA). We did not detect any correlation between the sedi- mentation rate and morphological and color variables. We found that UV chroma is the best predictor for the cutaneous immune activity. Indeed, male budgerigars with high UV reflectance in the breast feathers showed stronger immune responses. These results are consistent with the idea that UV colors are special signals con- veying information about a bird’s condition. Keywords Feather coloration Æ Ornaments Æ Phytohaemagglutinin Æ Sexual selection Æ Structurally based coloration Introduction Female mate choice is an important driving force in sexual selection (Darwin 1871). Females use various signals to assess the quality of males, including sexual vocalizations, behavioral displays, pheromones, and morphological traits (Andersson 1994). For signals to contain reliable information, they must be costly to produce and/or to maintain (Zahavi 1975; Grafen 1990). Females may ben- efit from mating with high-quality mates, because such individuals may provide ‘‘direct’’ (e.g., territorial space and the resources therein) or ‘‘indirect’’ (e.g., good genes) benefits (see Andersson 1994). Whatever the benefit gained, the most obvious cues used in mate-choice deci- sion are plumage ornaments, such as long feathers and brightly colored patches. Indeed, since Darwin (1871), plumage of birds has been an important focus for many evolutionary biologists interested in sexual selection. Many studies of birds have provided evidence that females can benefit by choosing a male according to the expression of colorful plumage ornaments (e.g., Andersson 1994). There is also increasing evidence that male mating pref- erences are based on female plumage ornaments, sup- porting the importance of plumage traits in sexual selection (e.g., Jones and Hunter 1993; Griggio et al. 2005). Plumage coloration can result from pigments deposited in feathers, such as carotenoids (giving feathers yellow to red coloration), melanins (brown and black coloration), or other pigments (such as are found in some parrot feathers, Masello et al. 2004; McGraw and Nogare 2005). Other than pigments, the coloration of feathers can be caused by nano-scale reflective tissues (structurally based coloration), usually producing ultraviolet (UV) and iridescent coloration (Gill 1995). One of the most controversial issues in evolutionary biology is the identification of costs of particular orna- ments used in mate choice. Many hypotheses try to answer the question: ‘‘Why should individuals have a sexual preference for exaggerated or particular colored traits?’’ Melanin-based ornamental colorations are under social control: in a number of species, melanin-based traits function as badges of social status (the ‘‘status sig- nalling hypothesis’’ Rohwer 1975; Senar 1999; Hoi and Griggio 2008). Moreover, recent studies suggest that melanins may not be as inexpensive to produce as initially M. Griggio (&) Æ V. Zanollo Æ H. Hoi Konrad Lorenz Institute for Ethology, Austrian Academy of Sciences, Savoyenstrasse 1a, 1160 Vienna, Austria E-mail: m.griggio@klivv.oeaw.ac.at Tel.: +43-1-515812753 Fax: +43-1-515812800 Ecol Res DOI 10.1007/s11284-009-0632-3