Summary
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 plumage on their breast than those in low-quality scrub. No other phenotypic differences associated with winter habitat were detected. Additionally, yearling males arriving earlier on the breeding grounds in Ontario had more extensively black breasts than those 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.
Methodology
Our field work during the nonbreeding season took place in high-quality (black mangrove, Avicennia germinans) and low-quality (second-growth scrub) habitats at Font Hill Nature Preserve, Westmoreland Parish, Jamaica, West Indies (18° 02′N, 77° 57′ W), during a period when wintering American Redstarts are stationary (22 October–18 November 2008). This period corresponds to the time shortly after the redstarts establish their winter territories, allowing us to investigate potential correlations between phenotype and territory establishment. During the breeding season (1 May–20 July 2008), our study site was the Queen's University Biological Station, Chaffey's Lock, Ontario, Canada (44° 34′N, 76° 19′W). American Redstarts begin to arrive on their wintering grounds from mid-to-late September through October, and they remain territorial through the entire nonbreeding period (Holmes et al. 1989, Marra 2000). In Jamaica, the early part of the redstart's wintering season is typically more mesic than later winter (Studds and Marra 2005). In Ontario, we surveyed daily to determine the arrival date of all males. We then ranked the birds' arrival, to account for missing data (n = 9) where the order in which the individual arrived was known but the precise date (within a 2- to 3-day window) was not. In instances of a tie in two individuals' arrival date or inferred arrival date (n = 4), we assigned arrival order by flipping a coin.
On both the breeding and wintering grounds, we captured yearling male American Redstarts with mist nets on their respective territories (determined via daily surveys both before and after the day of capture), using a combination of both passive netting and song playbacks accompanied by a decoy. We aged and sexed all captured birds (Jamaica: n = 17, Ontario: n = 22) according to Pyle (1997) and banded them with a unique combination of two or three color bands and either a U.S. Fish and Wildlife Service (Jamaica) or Canadian Wildlife Service (Canada) aluminum band. From each captured bird, we recorded the wing chord (mm), mass (g), tarsus length (mm), and tail length (mm). For plumage analysis, we plucked a single tail feather (rectrix 3), and took four or five pictures (Canon Powershot A460) of each individual in a series of standard poses in front of a gridded background. From males captured in Ontario, we also collected 2–3 mm of tissue from the central claw of each foot for use in analysis of stable carbon isotopes.