Hey, all.
Nick Moore got it. Yellow-breasted chat. The bird that can sing or say anything--anywhere, anytime. It was, as far as I could ascertain, a typical adult of the species. Nick also said: "Usually they start their more normal song but Ted may well be keeping the full recording from us." It is true that the bird eventually started to give a normal ("normal" being relative with the the crackup, clownish chat) song, but only after a long while of this consistent, stereotyped, down-slurred, wavering whistle.
Long ago, before the era of smartphones and small pocket recorders, I heard a chat in Gregory Canyon, Boulder County, giving only a slowly uttered, quite-low, monotone whistle. Was nearly certain I had a northern pygmy-owl. But no.
About a decade later, THIS chat was at Rabbit Mountain, Boulder County:
https://www.xeno-canto.org/184929
I think you'll see (and hear) why I thought I had a rare curve-billed thrasher. I wasn't the only one who thought that. :-)
Ted Floyd
Lafayette, Boulder County