Abstract
Male and female American goldfinches (Spinus tristis) express condition-dependent carotenoid-based plumage and bill coloration. Plumage color is relatively static, as pigments incorporated into feathers during the spring molt cannot be mobilized thereafter. In contrast, bill color is dynamic, reflecting changes in condition over short time periods. Previous studies have shown that male and female ornaments, though similar in expression, are differentially related to measures of immunocompetence, suggesting that the relationship between ornamentation and parasite infection may differ between the sexes. In this study, we evaluate the relationship between condition-dependent ornamentation (plumage and bill color) and blood parasite infection in male and female American goldfinches. We captured goldfinches after completion of the pre-alternate molt and prior to the onset of nesting and assessed prevalence of Trypanosoma parasites via blood smears. Plumage color strongly predicted trypanosome infection: Birds with more colorful plumage were less likely to present infections. In contrast, we detected no relationship between infection and bill color, which in other studies has been shown to dynamically reflect current condition. Sex did not affect the relationship between infection status and either ornament. Together, these results suggest that physiological pathways linking carotenoid ornamentation and infection may vary even within a single species.
Highlights
Ornamental signals used in social interactions often carry information about the condition of an individual (Zahavi 1975; Hamilton and Zuk 1982; Grafen 1990; Andersson 1994; Searcy and Nowicki 2005)
Sex was not related to infection status, nor was the interaction between sex and plumage color (Table 2, Fig. 2)
Neither bill color nor the interaction between bill color and sex was retained by the backward stepwise procedure, indicating they did not significantly predict infection status (Table 2, Fig. 2)
Summary
Ornamental signals used in social interactions often carry information about the condition of an individual (Zahavi 1975; Hamilton and Zuk 1982; Grafen 1990; Andersson 1994; Searcy and Nowicki 2005). Males and females exhibit similar ornamental expression (Tarvin and Murphy 2012), but expressed ornaments may be under different selection pressures in the two sexes, possibly leading to different signaling function for similar ornaments (Amundsen 2000; Amundsen and P€arn 2006; LeBas 2006; Clutton-Brock 2007, 2009; Kraaijeveld et al 2007; Murphy 2008; Tobias et al 2012).
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