The red-eyed vireo's song is a run of short two- and three-note phrases with a pause between each, repeated in slightly varied order. A male on territory delivers them steadily from the canopy through the day, including hours when other species have fallen silent, which is why the bird is a constant background of eastern summer woodland.
The often-quoted figure comes from a naturalist who counted the phrases of one male across a single day and reached a total above twenty thousand. It is a count of one bird on one day rather than a species average, and it is best treated as an illustration of persistence rather than a benchmark.
Two further points sit under the song. The repertoire is not one phrase but a set of distinct phrase types, so what sounds repetitive is more varied on a spectrogram than to the ear. And the red iris is an adult feature; young birds have a dark eye through their first winter, so a brown-eyed vireo in autumn is a young bird, not a different species.