We are witnessing the almost synchronous arrival of a guild of songbirds collectively called the leaf-gathering insectivores.
These birds include our vireos, warblers and tanagers. All earn their living by feeding on caterpillars and other herbivores that attack the leaves of deciduous trees. Leaf-gathering herbivores are friends of trees, gobbling up leaf-eating insects.
A cascade of events occurs in spring, allowing the return of warblers and vireos: leafing, then emergence of caterpillars, then arrival of leaf-gathering birds. In central and southern Maine, the first 10 days of May mark the arrival of many of these birds.
Among these arrivals are red-eyed vireos. I dare say that red-eyed vireos vie for the title of the most common forest bird in eastern North America. A treetop bird, the red-eyed vireo is much more often heard than seen.
Hearing red-eyed vireos is a breeze as they sing vigorously throughout the day. Their song is a series of two- and three-note phrases. An effective mnemonic to learn the song is “here-I-am, where-are-you, over-here, in-the-tree”.
The song is rather monotonous and dry. Despite the apparent monotony of their song, red-eyed vireos show remarkable diversity in their two- and three-note phrases. A typical red-eyed man sings about 45 phrases. These phrases are strung together to create a distinctive type of song. Each type of song consists of the same one to five phrases. A typical male sings around 30 types of songs.
A less common vireo breeding in Maine, the Philadelphia vireo, should be considered in this column. The red-eyed and Philadelphia vireos share an intriguing overlap in their songs.
The Philadelphia vireo closely resembles the red-eyed vireo but has a less distinct line above the eye and a yellow wash on the underparts. The Philadelphia vireo is also smaller, weighing an average of 12 grams compared to 17 grams for a typical red-eyed vireo.
Most breeding male songbirds defend their territory against other males of their species, but not against males of other species. However, red-eyed vireos and Philadelphia vireos defend their territories against their own species and against other species.
The song of the Philadelphia vireo is very similar to that of the red-eyed vireo. Even very experienced birders pass off Philadelphia vireos as the more common red-eyed vireos. The reason for the similarity will soon be apparent.
In the forests of northern New England, insect prey can become quite difficult to find during breeding season. Because both vireos eat the same insects, there is an advantage for a territorial vireo to keep a member of its own species and members of other vireo species away from its food sources.
In most cases, the vireos avoid direct confrontations on the limits of a territory. Instead, a territorial bird proclaims its ownership of a territory by singing from perches throughout its territory. Similarly, owners of adjacent territories sing throughout their territory. Neighboring birds recognize invisible but real boundaries, avoiding physical interactions.
The Philadelphia vireo’s problem is how to maintain sole ownership of a territory, defending itself against a larger and stronger red-eyed vireo that might try to expand its territory. Philadelphia vireos solved the problem by becoming a social mimic. These birds imitate the song of the red-eyed vireo.
Playback experiments have shown that red-eyed vireos cannot tell the difference between a red-eyed vireo song and a Philadelphia song. No wonder birdwatchers have trouble telling the two species apart by song! On the other hand, Philadelphia vireos can distinguish between a Philadelphia vireo song and a red-eye vireo song.
Philadelphia vireos mimic the song of the red-eyed vireo to level the playing field; it is a case of cheating on the muscles.
Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes comments and questions from readers to [email protected]
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