Robin singing that strange whistle as a whisper

Mid-December should be the time of the year when most birds are silent but this afternoon there was a robin in the neighbor's yard whisper singing. This is when birds practice their song but at a very low volume. So it was late for behavior. But what was even more interesting was that he was singing/calling this high pitch whistle. 

I had to laugh because I thought it was a nose whistle at first.