RESEARCH ARTICLE Relationships among plumage coloration, blood selenium concentrations and immune responses of adult and nestling tree swallows Michelle L. Beck 1, *, William A. Hopkins 1 and Dana M. Hawley 2 ABSTRACT In a number of taxa, males and females both display ornaments that may be associated with individual quality and could be reliable signals to potential mates or rivals. We examined the iridescent blue/green back and white breast of adult tree swallows (Tachycineta bicolor) to determine whether plumage reflectance is related to adult or offspring immune responses. We simultaneously addressed the influence of blood selenium levels and the interaction between blood selenium and plumage coloration on adult and nestling immunity. Selenium is a well- known antioxidant necessary for mounting a robust immune response but its importance in wild birds remains poorly understood. In females, the brightness of white breast coloration was positively associated with bactericidal capacity, but there was no association with blood selenium. In contrast, male bactericidal capacity was associated with an interactive effect between dorsal plumage coloration and blood selenium concentration. Males with bluer hues and greater blue chroma showed increased bactericidal capacity as blood selenium concentrations increased, while bactericidal capacity declined in greener males at higher blood selenium concentrations. In nestlings, bactericidal capacity was positively associated with nestling blood selenium concentrations and white brightness of both social parents. These results suggest that white plumage reflectance is indicative of quality in tree swallows and that greater attention should be paid to the reflectance of large white plumage patches. Additionally, the role of micronutrients, such as selenium, in mediating relationships between physiology and signals of quality, should be explored further. KEY WORDS: Bactericidal capacity, Micronutrient, Plumage coloration, Tachycineta bicolor, Cutaneous immune response INTRODUCTION In many animals, individuals display elaborate ornaments that are used as signals to conspecifics (Andersson, 1994). Because these ornaments are often costly to produce or maintain, only individuals in the best condition will be capable of possessing the most elaborate ornaments, making ornament expression a reliable indicator of phenotypic and/or genetic quality (Zahavi, 1975; Hamilton and Zuk, 1982; Grafen, 1990). In males, these ornaments appear to be maintained primarily by sexual selection because more ornamented males are preferred mates or are superior competitors, and as a result obtain more mating opportunities and greater reproductive success (Andersson, 1994). However, in a number of taxa, females display ornamentation that is also associated with individual quality (Amundsen and Pärn, 2006; Kraaijeveld et al., 2007). Traditional sexual selection likely plays a limited role in maintaining female ornamentation because female reproductive success is largely limited by their ability to produce young rather than their access to mates (LeBas, 2006; Tobias et al., 2012). Female ornamentation is more likely maintained by social selection, competition among conspecific females for resources other than mates (LeBas, 2006; Tobias et al., 2012). Because different selective pressures maintain ornamentation in each sex, it is possible that similar ornaments are related to different aspects of individual and/or offspring quality in each sex (Massaro et al., 2003; Kekäläinen et al., 2010; Kelly et al., 2012; Pickett et al., 2013). In many birds, both sexes display colorful plumage that may be produced by pigments deposited in feathers (carotenoid- and melanin- based colors) or by the feather microstructure (structural coloration). One aspect of individual quality that plumage coloration is frequently linked to is the ability to resist parasite infection or mount an immune response (Dunn et al., 2010; Kelly et al., 2012). While previous work focused largely on pigment-based, particularly carotenoid, coloration, recent work has shown that structural coloration may also be related to immunity or levels of parasitism (Doucet and Montgomerie, 2003; Bonato et al., 2009; Griggio et al., 2010). For example, in male satin bowerbirds (Ptilonorhynchus violaceus), brighter blue or greater UV chroma is associated with lower parasite loads (Doucet and Montgomerie, 2003). Male ostriches (Struthio camelus) and south polar skuas (Catharacta maccormicki) with larger or more colorful white plumage patches mount a stronger humoral immune response (Bonato et al., 2009; Hanssen et al., 2009). Finally, stronger cutaneous immune responses [response to phytohemagglutinin (PHA) injection] have been associated with greater UV reflectance of green plumage in budgerigars (Melopsittacus undulates) (Griggio et al., 2010). Only recently have studies examined the relationship between female coloration and immunity and have primarily focused on white plumage patches. In female common eiders (Somateria mollissima), smaller white wing bands are associated with immunosuppression (Hanssen et al., 2006), while larger wing patches are associated with lower parasite loads in Eurasian black-billed magpies (Pica pica) (Blanco and Fargallo, 2013). A greater number of white spots and spots with greater UV reflectance are associated with a stronger cutaneous immune response in female diamond firetails (Stagonopleura guttata) (Zanollo et al., 2012). It seems that in both sexes, structural plumage coloration may be associated with ‘good genes’ for greater immunocompetence or more colorful individuals may be in better condition and thus mount stronger immune responses. The plumage coloration of adults may also be related to immune responses of their offspring. For instance, white plumage coloration of male ostriches is positively associated with the humoral immune Received 15 April 2015; Accepted 26 August 2015 1 106 Cheatham Hall, Department of Fish and Wildlife Conservation, Virginia Tech, Blacksburg, VA 24061-0321, USA. 2 2125 Derring Hall, Department of Biology, Virginia Tech, Blacksburg, VA 24061-0406, USA. *Author for correspondence (beckmic@vt.edu) 3415 © 2015. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2015) 218, 3415-3424 doi:10.1242/jeb.123794 Journal of Experimental Biology