Songbirds can certainly alter their songs. When I did a breeding bird survey in Maine for a private company, I was given an instructional tape, and was told by the ornithologists involved that some warblers will start to sing more like their neighbors as the season goes on. For instance, an American Redstart giving a more Bay-breasted like song when a Bay-breasted is its neighbor and visa versa.
I do find that, to my ear, the Warbling Vireos in the mountains of Colorado/western Colorado consistently sound rather different from those well out on the plains at such places as Tamarack Ranch. I believe that sonograms have been obtained from these disparate locations in CO, and that the differences between the two races (per my understanding; perhaps Nathan Pieplow or Andrew Spencer have something to say about this) support the reality of these apparent differences.
Bruce's playback study is most interesting. However, at the apparent zone where these two meet, such as St Vrain State Park in western Weld County, I have (on multiple occasions) heard a Western consistently sing in one tree and an Eastern consistently sing not too far away. I could easily hear both, the differences were apparent and remained consistent. Of course, my ID could be wrong, but others (eg, Ted Floyd and Tony Leukering) have had similar experiences. I believe that sonograms of western and eastern WAVIs were obtained from the same location near Boulder last year.
None of this seems surprising to me. Eastern Colorado is an area where many eastern and western taxa meet (and sometimes greet!!) I would expect both WAVIs to breed in CO, and I'd expect the zone of contact to be somewhere between the foothills and Nebraska. It seems (as in "appears to be, might be, current evidence suggests") that the zone of contact (at least from Denver north), is around I-25 (give or take 30 miles to each side).
That there are two valid Warbling Vireo races is hardly disputed (except for those who consider them separate species), with the western race undertaking a molt migration and the eastern race not doing so.
Best Wishes
Steven Mlodinow
Longmont CO
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