Thanks for the unusual songs!
The warbler in question actually was singing from the tip-top of the deciduous canopy on the day Chris McPherson and I came across it. It never came down except for a faster-than-the-eye flyby, whereupon it returned to the top of the trees. A very confusing fellow, till Jeanne-Marie and I got him to come in closer on Friday.
Several more Black-throated Blues were singing on Friday and we did notice some other aberrant songs along that branch of the trail, though nothing that came close to the "culprit's" renditions.
A couple of years ago, there was a Savannah Sparrow at Cemetery Fields in Amherst singing a Grasshopper Sparrow-like song.
I've read that there is a large element of learning involved in many species songs.
Chris Sheridan
Nashua