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.