Secondary sexual traits in males are thought to convey information about the quality of the male to rivals and potential mates. Semi-static signals such as sexually selected plumage are thought to convey information about how well a male fared through a critical period, such as development or molt, but it is unclear if they should relate to his current condition during the breeding season. We examined the relation between epaulet size and two measures of immune function, levels of baseline corticosterone (CORT), and body condition in adult male Red-winged Blackbirds (Agelaius phoeniceus). Epaulet size was significantly negatively related to the bacteria-killing ability (BKA) of the blood but was not related to the phytohemagglutinin swelling response. There was no significant relationship between CORT and epaulet size, but there was a significant positive relationship between CORT and BKA. In addition, BKA was significantly positively correlated with date, suggesting a possible tradeoff with breeding-related activity levels. However, body condition was not related to measures of immunity, CORT, or epaulet size. Our results indicate that male Red-winged Blackbirds with larger semi-static signals experience reduced BKA during the breeding season and that baseline levels of CORT can be positively associated with measures of immune function.
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