A species profile of the Philadelphia vireo is a daunting task for anyone foolish enough to agree to write it. It is easily one of North America’s most poorly known and obscure birds. It is named after a city of no great importance to the species—it is an uncommon migrant in Philadelphia—and even experienced bird watchers have difficulty recognizing the species by its vocalizations. It seems to be one of those fringe species barely on the radar of casual birders, forever doomed to be overshadowed by the larger and much more abundant red-eyed vireo. Yet, I am about to argue that their very obscurity is an adaptation that makes it possible for Philadelphia vireos to survive and even to thrive in the face of this overwhelmingly successful competitor.
I spent five formative summers of my youth studying Philadelphia vireos at the legendary Hubbard Brook Experimental Forest in the White Mountains of New Hampshire. I had the great fortune to be hired by Richard T. Holmes to assist with a study of the bird community on a 25-acre plot in the beech-maple-birch forests that cover the slopes of these mountains. This bird community was a rising star in the birding world. The Philadelphia vireo, however, was a mystery—a ghost bird that seemed determined to remain as inconspicuous as possible.
A Research Challenge
We knew Philadelphia vireos bred on our main study plot, but they stayed so high in the canopy that we rarely saw them or caught them in ground-level mist nets. Their songs were so similar to those of the abundant, omnipresent red-eyed vireo that we had great difficulty mapping their territories by the locations of singing birds. What a great challenge for an undergraduate research project! Why do they stay so high in the canopy? Why do they seem to use only a few of the tree species in the forest? Why do they sound so much like the red-eye?
The first clue I had about the behavior of the Philadelphia vireo came when I saw one feeding a fledgling that had apparently fallen from its high-canopy nest to a small shrub near the ground. The parent was busily feeding the fledgling while guiding it back up into the canopy. While exploiting this rare opportunity to gather data on their foraging behavior (my main job as a field assistant), I saw a red-eyed vireo attack and start pecking the fledgling. The Philadelphia vireo parent counter-attacked the larger red-eye—the two birds locked their feet together and rolled on the ground as they pecked each other. Eventually, the red-eye gave up and wandered away. I was pretty impressed—the smaller Philadelphia had apparently prevailed. This interspecific aggression suggested that the red-eyed vireo was a potential problem for the Philadelphia vireo. Is the inconspicuous behavior of the Philadelphia an adaptation to minimize contact with the red-eye?
Before we can understand what makes this question so interesting, we must first understand something about the red-eyed vireo. There has never been a formal, global population estimate, but the red-eyed vireo surely is one of the world’s ten most abundant species. It breeds over much of North and South America in an incredible variety of forest, woodland, and scrub habitats. Its incessant song is one of the characteristic sounds of North American summers, and it is locally abundant even in hyper-diverse tropical forests of Amazonia and the Andes. The red-eyed vireo is an adaptable forager—it searches for caterpillars, but eats just about any insect while supplementing its diet with fruit when available. They are also extremely flexible in their foraging tactics—they can adapt to just about any foliage type—and even in their migratory behavior. Populations of red-eyed vireo show just about every migratory strategy known—Neotropical, Nearctic, austral, altitudinal, and intra-tropical. By any standard, the red-eyed vireo is a successful species, an ecological dominant, and a formidable competitor for any other species that moves through green foliage searching for caterpillars.
So how does the Philadelphia vireo cope with this formidable competitor? It has essentially the same foraging tactics and diet, and the two species inhabit many of the same forest types. They both breed in northern North America and winter in Central America.
On migration, they are almost as inconspicuous as they are on their breeding grounds. When they do sing, they sound a lot like red-eyes and often go undetected. My impression is that they seem to prefer younger vegetation when they migrate, just as they do when breeding; I often saw them foraging in edge and early successional habitats. In fall, they leisurely work their way southward until they hit the mountains of Costa Rica, and Panama where they seem to have a largely red-eye-free refuge and can just be themselves for a few months.
Because it is so much (about 30 percent) smaller than the red-eye, the Philadelphia vireo is at a distinct disadvantage compared with its ecological superstar cousin. Smaller bird species generally lose fights with larger species, which means that it is important to avoid outright aggression. Philadelphia vireos are also less flexible than the red-eye in their foraging behavior. They are much more restricted to younger forests, which are often rich in caterpillars, than are red-eyes, which also thrive in young forests but can also persist in dense, dark, old-growth forests of beech and sugar maple, two tree species with leaves that are resistant to insects such as caterpillars.
Advantages of Mimicry
The Philadelphia vireo, therefore, faces a severe challenge—how can it coexist with an abundant ecological and behavioral dominant species that shares essentially the same niche? The answer provides one of the most fascinating examples of how behavioral flexibility can allow a subordinate species to coexist with a dominant one. As we learn more about the taxonomy of birds, we are coming to appreciate the role of deception in maintaining biodiversity. Many species that we thought were closely related because they look so similar are turning out not to be closely related at all. They are apparent examples of the evolution of mimicry, in which one species converges on the plumage and vocalizations of another. Examples of this process are numerous—downy and hairy woodpeckers, for example, have almost identical plumage but are not closely related.
One possible explanation for mimicry is that it might enable smaller species to compete successfully against larger, dominant species through trickery. Smaller toucans, for example, may be able to enter fruiting trees defended by larger species because they look exactly like their larger relatives, and would therefore be tolerated as if they were the same species. Plumage mimicry, however, would not necessarily work well for birds such as vireos, which defend exclusive territories mainly by singing. Such species would, if anything, be less likely to tolerate another species that had identical plumage. Vocal mimicry, however, might work very well.
Jake Rice studied the Philadelphia vireo in the early successional and bog forests of Canada and concluded just that—Philadelphia and red-eyed vireos were “interspecifically territorial,” meaning that they defended non-overlapping territories and responded to each other’s songs as if they were the same species. There were subtle differences in their songs—the Philadelphia often sang a little faster, or sometimes slower, but there was a great deal of overlap in all features of their songs. Rice concluded that the subordinate Philadelphia vireo was able to coexist with the red-eye in its favored habitat—northern bog forests dominated by alder—by mimicking its songs. The red-eye did not seem able to tell the difference between its song and that of the Philly; they attacked songs of both species with equal ferocity during playback experiments. Philadelphia vireos, however, behaved very differently to the song of the red-eye—they continued singing, but avoided directly approaching the audio speaker. It would make sense for them to avoid direct contact with the larger red-eye—why confront a larger competitor when you can trick them using by imitating their song? Bird watchers who have difficulty identifying Philadelphia vireos by song should not feel ashamed—even red-eyes cannot recognize them.
At Hubbard Brook, however, the situation was not nearly as straightforward. In this taller, more complex forest, the territories of the two species overlapped broadly, with the Philly mostly staying high in the canopy, mostly in ash trees, and the red-eye foraging more at middle heights. At Hubbard Brook, Philadelphia vireos occasionally sang songs that were similar to the red-eye, but they sometimes used a song that was clearly different. This song often repeated the same exact phrase many times in a row—a direct violation of the guiding principle of red-eye song: never, ever repeat the same phrase twice in a row. The song of the red-eye may seem monotonous and dull, but it is actually marvelously complex and variable. It sounds like a conversation. The Hubbard Brook Phillies, however, sometimes simply repeated the same phrase for long intervals.
These songs, like the bird, were maddeningly difficult to locate. It took me more than a month to confirm that this song belonged to a Philadelphia vireo. Unfortunately, I never recorded it or did playbacks. To me, it seems likely that this song was designed to communicate with other Philadelphia vireos without attracting the attention of the red-eyes whose territories overlapped their own. Alas, I never did any playback experiments to determine if red-eyes would ignore this slow, repeated song.
The only time the Philadelphia vireo seemed to interact aggressively with the red-eye was during the period when their fledglings left the nest, a period, when young birds are extremely vulnerable. During these few days, Philadelphia vireos defended a small space around the fledglings against red-eyes. There was no way to avoid a red-eye on overlapping territory. The Phillies had to attack them and hope that the red-eye would not care enough about them to fight back very hard.
More Research Required
Although we need a lot more study (researchers always say this), the interactions of red-eyed and Philadelphia vireos may not be unusual in bird communities. We are beginning to recognize that birds are much more intelligent than we had ever realized. Deceptive behavior that is “context-dependent” (used only under certain sets of conditions) was thought to be restricted mostly to us higher apes. Yet, here we have a small, obscure bird species that seems able to coexist with an ecological dominant over a large breeding range by pretending to be the same species.
The Philadelphia vireos are long gone from the Hubbard Brook plots since I worked there in the 1970s. They seem to depend upon early successional trees such as birch, alder, and ash that are replaced by beech and maples as forests grow older. Their go-to nesting habitat may be alder bogs in the far North where their preferred trees remain even as forests age. Although Philadelphia vireo is not an especially high priority for conservation right now, it is so poorly known that the species is potentially vulnerable.
What to Look and Listen For
At 5.25 inches in length and with an 8-inch wingspan, the Philadelphia vireo is similar in size and appearance to the warbling vireo: drab, gray to grayish-olive with a pale eyebrow and a dark line through the eye. The Philadelphia vireo, though, has a yellow throat and underparts, but much a paler yellow than the yellow-throated vireo. It is noticeably smaller and shorter-billed than the red-eyed vireo, which shares its niche. The Philadelphia vireo has a short tail and proportionally smaller bill than the red-eyed, with less distinct markings on the head. Sexes are indistinguishable (to humans). Its song is very similar to the red-eyed vireo but higher pitched, weaker, and choppier, often with log pauses between phrases. It often repeats phrases, unlike the red-eye.
Where and When to Look
The Philadelphia vireo nests farther north than any other vireo species in North America: across Canada’s prairie provinces, east to Newfoundland, and in the United States in the northernmost Great Lakes area and northern New England, where it can be found from late May through mid-September. On migration from mid-May through mid-June and late August through late September, it can be found east of the Great Plains. It winters in Central America. It prefers the middle to upper canopy of early to mid-successional deciduous forests, where it is often concealed by foliage.
Primarily an insectivore, the Philadelphia vireo also consumes fruit in fall and winter. Either in flight or perched, it picks caterpillars and other insects from foliage in the upper canopy.
Courtship and Nesting Behavior
The female Philadelphia vireo selects the nest site, usually near the top of a deciduous tree. She uses grasses and bark to construct an open, cup-shaped nest suspended from small, forked branches. Her mate often accompanies her, vocalizing. The nest is neatly lined with white pine needles and dried grass. She typically lays 4 eggs, white with a few dark spots, and both parents share incubation duties soon after the first egg is laid. Incubation lasts 11 to 13 days, and eggs hatch asynchronously. Both parents brood and feed the young. Fledging occurs in about 14 days, and parents care for their offspring for at least 10 days. —Dawn Hewitt