Philadelphia Vireo
Philadelphia Vireos imitate the songs of Red-eyed Vireos. Red-eyed Vireos do not discriminate among the songs of the two species, but Philadelphia Vireos do.

Philadelphia Vireos are small, unassuming songbirds of northern deciduous forests. Though widespread and abundant, they are easy to mistake for other similar species, or to overlook altogether in the dense, shadowy canopy. As a result, Philadelphia Vireos, even where relatively common, often go unnoticed in their breeding range. Many birders in the United States have seen and heard the species only in migration, when it is widespread and often rather conspicuous, though still not nearly as common as other vireos.

Philadelphia Vireos have the most northerly summer range of all vireos. They breed across much of Canada, from eastern British Columbia through the prairie provinces to the St. Lawrence River basin. During spring and fall migration, they occur widely throughout the United States. (Despite their name, coined by John Cassin after a specimen collected in 1851, they are uncommon in Philadelphia.) They winter in Central America, from southern Guatemala to central Panama.
Philadelphia Vireo Range Map

The life history of Philadelphia Vireos is strongly affected by interactions with Red-eyed Vireos. The two species closely resemble each other, especially in habits, voice, and distribution. Their breeding territories often overlap, and the two species frequently show aggression against each other as well as their own species. The foraging specializations of the two species reinforce their coexistence; in tall, older forests, Philadelphia Vireos tends to forage higher in the canopy than Red-eyed Vireos.

Philadelphia Vireos can also imitate Red-eyed Vireo song. Research has shown that Red-eyed Vireos do not distinguish the two species by song, whereas Philadelphia Vireos do. Through song, then, Philadelphia Vireos can establish and maintain territories without direct encounters with Red-eyed Vireos, which are larger and tend to prevail in physical confrontations.

Philadelphia Vireos feed primarily on insects and insect larvae gleaned from living leaves high in the treetops. Philadelphia Vireos may pick insects from leaves either while the birds are in flight, or while perched. In fall and winter, Philadelphia Vireos eat berries and seeds.

Like other vireos, Philadelphia Vireos typically build their nests high in the tree canopy, suspending the structure in the fork of a horizontal branch. Nests are made of grasses and bark strips, held together with beard moss (Usnea). Clutches usually consist of four eggs. Brown-headed Cowbirds commonly parasitize Philadelphia Vireo nests.

Description: Philadelphia Vireos are small songbirds, measuring about 5 inches in length. Sexes are similar. Upperparts are drab olive green, with no wingbars; underparts are yellow, brightest on the center of the throat. Cap is dusky gray-green. Lores (area between eyes and bill) are dark, showing distinct contrast with white eyebrow. Bill structure is typical of vireos, with very subtle curvature near the end of the upper mandible, ending in a small hook.

Philadelphia Vireos closely resemble Warbling Vireos, but the latter species is larger, with pale lores and a broader but less distinct eyebrow stripe. Tennessee Warblers also are somewhat similar in appearance, but have thinner, sharply pointed bills.

Voice: Songs, issued by males only, take several forms. One common type closely resembles the basic song of Red-eyed Vireo, with short, unrepeated musical phrases separated by long pauses. On average, Philadelphia Vireos sing this song more slowly than Red-eyed Vireos; pauses in the song of the former species last about three seconds, and in the song of the latter, about two seconds. Philadelphia Vireos also sing a distinctive song consisting of the same phrase repeated over and over, with pauses each time after the phrase is uttered. Different males use different phrases. Call is a soft “ehhh.”


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