I woke this morning to the single-note call of a Townsend's solitaire, but yesterday morning one was doing his whole spring song routine. Someone once told me that autumnal singing is related to a minor hormone surge at this time of year -- something that I need to research.
But that single note is one of our key seasonal indicators. M. says that it is actually her favorite bird call. I always think of it as the "All Clear" following a snow squall -- that or the calling of chickadees.