Paula Sullivan wins the prize for quickness and bird-smarts.  She informed me that my mystery bird might be an Indigo Bunting, based on his song style and habitat.  And after listening to several recordings, I think she is right!  Although, my bird had a shorter trill at the end of his song than on the recordings I have found.  Perhaps that part got lost in the wind, or maybe he just had a different "accent".
Thanks to Paula!
Jenny