Hamilton and Zuk (1982) proposed that bright plumage in birds evolved as an advertisement of parasite resistance in response to heavy parasite loads, and they predicted that sexual dichromatism should be strongly correlated with parasite loads across species. To test their hypothesis, we sampled 935 individuals of 19 species of passerine birds in Louisiana for the presence of blood Nest height, a variable generally not considered in most recent literature on the Hamilton-Zuk hypothesis, was as good or better a predictor of para- site prevalence as was sexual dichromatism or plumage brightness. Because certain ornitho- philic vectors are most common in the canopy, the relationship between nest height and para- site prevalence may follow from the natural history of parasitism. Ecological conditions may influence blood-parasite loads in the species studied, suggesting that genetically based resis- tance is less important. If parasite vectors are more common in the canopy, then more colorful bird species will be more heavily parasitized, on average, than less colorful species, because bird species that live high in the trees tend to be more colorful than that live closer to the ground. Received 18 July 1994, accepted 14 August 1995. HAMILTON AND ZUK (1982) predicted that bird species with strong plumage dichromatism also will have the highest prevalence of parasitic in- fection. They indicated that species with heavy parasite burdens have undergone strong selec- tion for ways to advertise relative healthiness, such as by evolution of bright plumage. They proposed that males of such species would ex- perience strong sexual selection, generating strong sexual dimorphism in plumage. Hamil- ton and Zuk's specific prediction was that those (species) with most evident sexual selec- tion are most subject to attack by debilitating parasites. Their review of data on blood para- sites in 109 species of North American passer- ines revealed that bright plumage was posi- tively associated with prevalence of blood para- sites, a finding consistent with predictions of their hypothesis. They concluded that although alternative explanations for their results could be offered, their results suggested that genetic variation for disease resistance contributes to sexual selection and evolution of bright plum- age.