Western Warbling Vireo: A Newly “Discovered” Cryptic Species

_{This YouTube video was produced by the American Bird Conservancy.}

The Western Warbling Vireo (Vireo swainsoni) is a quintessential species of spring and summer in the western half of North America. It was recently split from the Eastern Warbling Vireo; before 2025, the two were considered a single species: Warbling Vireo. Researchers determined that there were, in fact, two separate species, based on behavioral and genetic differences.

This rather drab bird is often hard to spot, hidden up among the leaves of tall deciduous trees, but its cheerful, energetic song is hard to miss. Males do most of the singing, but females sing as well — an unusual trait among temperate songbirds. Also unusual is this bird’s habit of singing while sitting on the nest, males in particular.

Western Warbling Vireos are frequently parasitized by Brown-headed Cowbirds, which lay their own eggs in the nests of other species, often resulting in the death of some or all of the host’s young. When they lay in a Western Warbling Vireo’s nest, the entire brood is usually lost.

Cowbirds seem to cue in on this vireo’s habit of singing from the nest, as birds that sing more on the nest are more likely to be parasitized. Female cowbirds are quite crafty, even using the movements of parent birds to determine the location of a nest. The more often a parent uses the same paths to and from the nest, the more likely cowbirds are to find it. Unlike the closely related Eastern Warbling Vireo, the Western species does not typically remove cowbird eggs. As a result, local vireo populations can decline when cowbird populations are high. In some areas, ornithologists fear that populations of Western Warbling Vireo may be driven to extinction.

Also known as: Vireo Gorjeador Occidental (Spanish), Vireo Cantor (Spanish)

Learn more at https://abcbirds.org/birds/western-warbling-vireo/