One of the most common breeding songbirds in the woodlands of eastern North America, the Red-eyed Vireo is more often heard than seen. This species' persistent song is heard throughout the day: cherr-o-wit, cheree, sissy-a-wit, tee-oo. Indeed, the unending and monotonous character of its song prompted Bradford Torrey in 1889 to reflect wryly, “I have always thought that whoever dubbed this vireo the ‘preacher' could have had no very exalted opinion of the clergy” (1: 343).
As with many Nearctic–Neotropical migrants, the evolutionary origin of the Red-eyed Vireo can likely be traced to the Neotropics (2). This species overwinters principally in the Amazon basin of South America east of the Andes and breeds extensively in eastern North America and west across Canada and the northern United States. The sexes are weakly dimorphic and socially monogamous. The female builds the nest, incubates eggs, and devotes more time than the male to brooding and feeding of young. This species is largely insectivorous during the breeding season, when they are most often observed foraging in canopy vegetation. During the nonbreeding season, fruit is an important part of the diet, especially in migration and tropical overwintering habitats. A mixed diet of fruit and insects is especially conducive to fat deposition during migration. The Red-eyed Vireo is a nocturnal migrant whose magnetic compass figures prominently in its orientation during intercontinental flight (R. Sandberg, J. Bäckman, M. Lohmus, personal communication).
The Red-eyed Vireo is part of the subgenus Vireosylva and the larger Vireo olivaceus superspecies complex, which includes Yellow-green Vireo (V. flavoviridis) of Middle America, Black-whiskered Vireo (V. altiloquus) of southern Florida and the West Indies, Yucatan Vireo (V. magister) of Yucatan Peninsula and nearby islands, Chivi Vireo (V. chivi) of tropical lowland South America, and Noronha Vireo (V. gracilirostris) of Fernando de Noronha, Brazil. Evolutionary relationships within this complex have challenged taxonomists for decades. For example, V. flavoviridis and V. altiloquus were lumped into V. olivaceus for many years, but eventually were generally treated as species. Despite those splits, V. chivi continued to be lumped with V. olivaceus until phylogenetic analyses revealed paraphyly between the North American and South American lineages of V. olivaceus with respect to V. altiloquus (3, 4), leading to the recent taxonomic split of V. olivaceus and V. chivi (5).