Steven Mlodinow writes:

>I once heard an Oregon Junco in June giving a buzzy song on Whidbey Island that most resembled a Black-throated Blue Warbler. It is hard to know what caused it to sing this way, but in that case, I was able to feel pretty assured it wasn't a hybrid

Actually, I can tell you from experience that is the song of either
a.) a first-year non-territorial male junco whose hormonal levels
aren't up to generating a full trill or, as likely, b.) a male whose
young has fledged and whose hormonal levels are dropping, resulting in
a buzzy, drier song suggesting (most often) Clay-colored Sparrow
(Spizella pallida) or, sometimes, Black-throated Blue Warbler
(Dendroica caerulescens).

Michael Price
Vancouver BC Canada
loblollyboy at gmail.com

Every answer deepens the mystery.
- E.O. Wilson