Vireos of brotherly love...

Two Philadelphia Vireos turned up in the yard this morning, a regular May visitor here. One of the two was singing; in spite of what some field guides say this species does sing during spring migration. I've tried for many years to nail down one single characteristic that will separate Philadelphia and Red-eyed Vireos by song. I haven't isolated anything that is black/white; I'd be thrilled to hear if anyone else has. The standard field guide description as "higher, thinner, slower" is unreliable: On a sonogram the pitch of the two species is actually just about the same; the difference in quality ("thinner") is very subtle and hard to judge, and the pacing of both species varies enough that the amount of overlap is huge. The best tag I have found is that a Philadelphia is more likely to begin a phrase with two quick high pitched notes before the short warble: "tsetse-widdlewaddle" where the Red-eyed would usually just say "widdlewaddle." It's not a hard and fast thing however, as I've also heard Red-eyeds sing a "tsetse" intro sometimes, and the Philadelphia generally only uses it on some phrases, not all (or even most). Still, repeatedly hearing the "tsetse" is my flag to pay closer attention and try to see the bird, often yielding a Philadelphia in the end.

Any thoughts?