This is a Mockingbird's "whisper song", sung in mid October. Mockers and other songbirds sometimes sing these "whisper songs", which can only be heard a short distance away (50 feet or less) in the latter parts of the year. This recording is from only 10 feet with a very sensitive shotgun mic. Female mockers are known to do this, but the reasons are not clear. If you have a theory, let me know!
Douglas Von Gausig (recordist; copyright holder), Naturesongs.com
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.