Delayed maturation of secondary sexual signals in first-year male American redstarts
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Male birds of many species use conspicuous song and plumage displays in both courtship and territorial interactions. In some species, one or both of these signalling traits may not reach full adult maturity until a male’s second year of life. While the prevalence of delayed plumage maturation is well documented, delayed song maturation may be more difficult to detect. As a result, there are few studies which report age-based song differences between first-year and adult males. Additionally, despite the potentially large degree of variation of each trait within yearling males, little work has examined the benefits for young males who appear or sound more adult-like. Here, I investigate variation in both song and plumage displays of yearling male American redstarts (Setophaga ruticilla) as they relate to success during the breeding and non-breeding seasons. I first demonstrate a relationship between the degree of adult-like black plumage and both non-breeding season habitat quality in Jamaica and breeding season arrival date in Ontario. Previous studies have linked breeding season arrival date with winter habitat quality in adult males using stable-carbon isotope analysis. Together, these results suggest that variation in yearling male appearance may signal an individual’s competitive ability for high-quality resources. Next, I quantified the mate-attraction songs of both adult and yearling males and demonstrate a delayed maturation in this song type. I also present evidence of the potential benefits of expressing a more adult-like song by linking song structure with reproductive success in adult males. Finally, I demonstrate a potential relationship between the degree of adult-like song and plumage expression in yearling males, but not adult males. This work demonstrates that the delayed maturation of sexual signals may play an important role in the life-history of yearling male American redstarts, and highlights the need for in-depth analyses of individual variation of multiple sexual signals in this poorly-studied age class of birds.