American Robin whisper song

This is a recording of the whisper song of an American Robin, a male that sang quietly a few feet from me without opening its beak; the only sign that it was singing was the movement of its throat. The song had a ventriloquial sound to it, and at first I looked around to find the bird that was singing, before I realized that it was the lone bird perched right in front of me.
The whisper song differs in several ways from the normal song. There's more repetition, the rate of delivery is faster and the phrases are shorter. Like the dawn song, there are no breaks in the song and there are numerous "hissellies". Sometimes the whisper song is made up entirely of hissellies, (see Thrush songs), and mimicry has been described, but I couldn't detect any in this song.
Many species sing whisper songs, but their function seems to be unknown. They are associated with male-male aggression, or in close interactions with a mate, and perhaps may used when the bird wants to remain relatively anonymous as when near a predator or rival, or close to the nest (1.00pm, Mar, PA).

The first 33 seconds of phrases:
































The notes are delivered at a rate of about 104/min. There are 18 different song phrases, 8 hissellies, and 4 different call notes, including a flight call at 27 secs, the only time the bird opened its bill.

References:
Subsong vs. Whisper Song (Earbirding)

Vocal copying by American Robin (Sibley Guides)