Compare this western Warbling Vireo to the eastern one just below it. The latter was the bird for May 17. Both songs have several peaks (pointed high notes) distributed roughly evenly through the song, but the western form has fewer, longer, notes in between. This gives the eastern song an orderly appearance, while the western looks herky-jerky.
The differences you see are representative of the songs of the two populations. They have been proposed for separate species status, but the authorities have so far decided against it. That could change. But it really doesn't matter whether two genetically-similar populations that are very similar looking and sounding, and that use similar habitats, but live in different places, are called different species or not. Until their ecological needs are different enough for them to live in overlapping ranges, they have not attained the full measure of specieshood. The two Warbling Vireos, the two meadowlarks, the Spotted and Eastern Towhee, all fit this pattern. But, regardless of the taxonomic rank we have bestowed (imposed?) on them, they all exemplify evolution in action.