Arguably one of the most common songbirds breeding in the woodlands of eastern North America, the Red-eyed Vireo is more often heard than seen. The persistent, if not enthusiastic, song is heard throughout the day: cherr-o-wit, cheree, sissy-a-wit, tee-oo, and on and on. The song's unending and monotonous character 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” ( Tyler 1950b : 343).
As with many Nearctic-Neotropical migrants, Red-eyed Vireos are thought to trace their evolutionary origins to the Tropics ( Cicero and Johnson 1998 ). This species breeds extensively across Canada and eastern North America; these populations, the focus of this account, winter principally in the Amazon basin of South America east of the Andes. Other populations currently classified as Red-eyed Vireo are resident (tropical regions) or migratory (subtropical to temperate regions) in South America. In all populations, 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. These birds are 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 tropical winter quarters. 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, and M. Lohmus pers. comm.).
Important information is available on communication ( Lemon 1971 , Barlow and Rice 1977 , Borror 1981 ), breeding biology ( Lawrence 1953a , Southern 1958 , Barlow and Rice 1977 ), foraging behavior ( James 1976a , Robinson and Holmes 1982 ), and migration ( Loria and Moore 1990 , Sandberg and Moore 1996 ).