Male American redstarts (Setophaga ruticilla) exhibit delayed plumage maturation. In other words, the first-year males look a lot like the olive-colored adult females. Males don't acquire their characteristic glossy black coloration and bright orange wing, tail, and flank patches until after they molt at the end of their first breeding season.
But sometimes younger males begin acquiring their adult colors before the molt. Scientists at the Smithsonian Migratory Birds Center and Queen's University wondered if this premature coloration influenced the ability for yearling males to acquire high-quality winter habitat. In other words, could looking more like an adult give the younger males an advantage?
Adult male (left), young male with early molt (center), and female (right) redstarts.
© Cindy Mead, Ryan Germain, and Gerhard Hofmann.
Males who occupy high-quality, mangrove winter habitat are in better physical condition, arrive at their breeding grounds earlier, and have greater reproductive success than those found in low-quality, scrub habitat.
Naturally, with all these benefits, there is great competition for the high-quality winter sites. So how does a yearling male manage to grab a slice of that good habitat for himself?
In order to find out, the team of scientists studied American redstarts in their winter territory in Jamaica as well as their breeding territory in Ontario, Canada. The researchers captured yearling male restarts in mist nets in both locations. They banded the birds, recorded physical measurements, and took samples for plumage and stable carbon isotope analysis.
Lush, green habitat on left is mangrove;
drier, more open habitat on right is scrub.
The yearling males captured in the high-quality mangrove habitat in Jamaica had more black plumage on their breasts than the birds living in the lower-quality scrub habitat. The team also found that the yearlings with more black breast plumage arrived on the breeding grounds in Ontario before their olive-colored counterparts.
From their findings, the team was able to deduce that yearling males with more black breast plumage held an advantage over the yearlings without black plumage. These darker males were able to occupy a higher quality habitat in the winter, providing themselves with a more reliable food source and raising their chances of survival. They also departed for the breeding grounds before the yearlings in the scrub habitat, which gave them a reproductive edge over the competition.
In many species of birds, a dark plumage badge is often associated with competitiveness and dominance, so the black chest plumage in yearling redstarts may be a sign of aggressiveness. The yearlings might even be acquiring the early black plumage as a result of feathers lost during fights over territory. In short, the yearling redstarts may take advantage of their premature adult plumage by using it as a signal of quality, allowing them to compete with the adult males for the most desired resources and habitat.
This article summarizes the information in this publication:
Germain, Ryan, Marra, Peter P., Kyser, T. Kurt and Ratcliffe, Laurene. 2010. Adult-Like Plumage Coloration Predicts Winter Territory Quality and Timing of Arrival on the Breeding Grounds of Yearling Male American Redstarts. The Condor, 112(4): 676-682.
The quality of winter territory can have important consequences for migratory songbirds throughout the year. In the American Redstart (Setophaga ruticilla), a warbler in which plumage maturation is delayed, yearling males winter in a variety of habitat types that vary in quality. Little is known regarding which physical traits allow some yearlings to occupy higher-quality sites. Here, we measured eight variables characterizing the plumage and morphology of yearling males in two habitats that differ in suitability to determine which aspects of phenotype predict winter habitat occupancy. Yearlings wintering in high-quality mangrove habitat in Jamaica had more extensive adult-like black plumags on their breast than those in arriving later. Previous studies using stable carbon isotopes have linked adult male American Redstarts' date of arrival in the breeding range with quality of their winter habitat. Our findings indicate an association between the extent of adult-like plumage and habitat occupancy, suggesting that variation in yearling males' appearance may be correlated with their ability to compete for high-quality habitat.
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