The red-eyed vireo is a small American songbird, measuring between 4.7 and 5.1 inches (11.9 and 12.9cm) in length and having a wingspan of 9.1 to 9.8 inches (23 to 25cm). Its body is olive-green with a whiter underneath and a blue-gray crown. It is characterized by dark red eyes, white eyebrows, and a black stripe through the eye. Young red-eyed vireos have brown eyes and yellower underparts. These birds sing continuously and are more often heard than seen. Their song is a broken series of slurred notes. Each phrase usually ends in either a downslur or an upswing, giving the impression that the bird is asking a question and then answering it, over and over. Red-eyed vireos glide through the woods with incredible speed, especially while in pursuit of a rival.
The red-eyed vireo is a Neotropical migrant, breeding in deciduous and mixed deciduous forests in North America and migrating to South America to spend the winter months. Like most songbirds, red-eyed vireos migrate almost entirely at night. They are more abundant deep in the forest, but are also found in urban areas and parks with large trees.
This species feeds on insects, especially caterpillars, and small fruit. Red-eyed vireos diligently search for insects in the tree foliage, moving up and down branches and carefully examining each leaf, bud, or blossom. Although animal food accounts for 85% of the diet of red-eyed vireos, these birds may adapt their diet to be completely frugivorous (or fruit-eaters) during the winter season.
Red-eyed vireos have been one of the most abundant and widespread birds of North America, although their population has recently declined. Forest fragmentation and destruction of both its northern breeding habitats and tropical wintering habitats has a great impact on the red-eyed vireo population. These songbirds are also vulnerable to predation by cowbirds that lay their eggs inside the nests of other bird species. Their declining populations are of particular concern because they so effective at controlling insect pests.