[Posted by Chuck Almdale]

David Sibley has written a long, detailed article about distinguishing this recently split species pair in the field. Read the article, complete with illustrations, sonograms and range maps, to find out all about it.

His lead paragraph:
The Warbling Vireos were considered a single continent-wide species until 2025, when two species – Eastern [Vireo gilvus] and Western [Vireo swainsoni] – were officially recognized by AOS and eBird (Cicero 2025). The differences between these populations have been the subject of intensive research for decades, mainly in Alberta where they meet. I have spent a lot of time studying their identification over many years, with new focus in the months after the split.

His Quick Summary:
In practice virtually all will be identified presumptively by range. Positive identification depends on careful analysis of details of song (but questions remain about variation in song). Positive identification of silent birds is not possible on current knowledge.

According to the article’s range maps, their breeding ranges overlaps in Alberta. South of there they are divided by the crest of the Rocky Mountains. According to Sibley’s list of vagrants, the “only fully convincing record of an Eastern Warbling Vireo west of the Rocky Mountains (the only one supported by recordings of song)” was in Pima County, AZ on 30 June 2014. And…after listening to the sonograms, my decrepit ears hear the eastern song as having clearer, more musical and less buzzy tones than the western song. Songs can be the same 3 1/2-second length.

So…no more reporting of ‘Warbling Vireo sp.’ as these two are so easily differentiated, as anyone can see from the above illustration. Right, birders? Right. And stay out of Alberta during breeding season lest you be driven mad.