Here’s a recording I made today of a couple of neighboring Bell’s Vireos in western Franklin county, Ohio. By location, these “should’ be Eastern Bell’s Vireos (the nominate race, Vireo b. bellii), but can we back that up with these recordings?
I actually don’t know the answer to that question! Check back soon for an update. I’ll hopefully get a chance this week to browse some of the online archives and poke around in the published literature a bit to see what, if anything, is known about subspecific differences in song. If you know the answer, please let me know in the comments!
Update (14 June 2012):
As mentioned by Nathan Pieplow in the comments below, it sounds like it is currently not possible to distinguish Bell’s Vireo subspecies by song. You can read more about Bell’s Vireo subspecies (especially the Least Bell’s Vireo) here.