Chemical contaminants in the environment can influence both morphological and behavioral traits. Ornamental traits such as plumage color may be especially valuable in detecting the effects of toxic chemicals in the environment, although they have been little used to date. We examined patterns of plumage color in subadult female Tree Swallows (Tachycineta bicolor) breeding in an area on the Hudson River that had high levels of PCB contamination and compared them with specimens from other parts of the species' range and with data from two previous studies of plumage color. Tree Swallows are one of the few species of birds where females, but not males, have a distinctive subadult plumage during their first breeding season. Females from four breeding colonies in contaminated areas had significantly more adult-type blue-green coloring than females from the rest of the species' range. Subadult female plumage color at these sites formed a continuum between the normal dull brown plumage found elsewhere and the blue-green plumage of older females. Among these subadults, more colorful individuals bred earlier, and earlier breeding in turn led to larger clutches. The patterns of plumage anomalies described here are consistent with disruption of the endocrine system resulting in the early expression of an adult trait. This study supports recent suggestions that examining variation in ornamental traits of animals is an efficient means of detecting effects of chemical contaminants in the environment.