Birds singing strangely

Some years ago, I was in my small London garden in the dead of winter when a male blackbird landed on a vine post near me and started to sing.
Blackbirds do not sing in wintertime, but this one was not technically singing.
It went through its entire summer repertoire, quietly and under its breath. I stood transfixed at this quiet cascade of notes pouring out from a blackbird that did not even bother to open its beak.
Now much the same thing has happened with a robin.
We know our two robins well. They come into our shed to eat maggots and small morsels of Cheddar cheese, sometimes from my knee if I feel like it. Now they are mostly away, seldom appearing as they obtain new feathers in the annual moult.
In their absence a new robin has appeared on the block. He is a bright, upright robin, who wants company more than food.
He has taken cheese from flagstones beneath me as I crumbled it into small pieces and let them fall. He dashed in to eat them so close to my feet that I dared not move for fear of treading on him. He is brave.
Two days before writing this, Margreet and I heard what sounded like young birds singing in the distance, possibly all in line on a TV aerial, or so we imagined. We accepted these distant songs and took pleasure in them.
Then our new robin came to sit on a wooden sculpture a few feet away from where we were sitting. And, like the blackbird in mid winter, this robin was singing under his breath - in mid summer. The notes might have come from a long way away.
His repertoire of songs must have lasted for 15 minutes, and stopped only when a nearby blackbird shouted out her warning signal call.
A friend arrived the following day to witness and hear the same robin singing the same songs from the same perch.
I passed close to the bird to fetch cheese bits, which he declined to eat, not moving, and singing all the while.
Our own human conversation ended as we watched the bird and listened to his song – for perhaps five minutes.
The new robin is clearly very friendly, wants company, is fearless of people, and has songs to whisper that he wants us to hear.
We dearly love our regular pair of robins, but we also hope that this songster comes to stay. But robins are very territorial – ours particularly so. I imagine that there will be fighting before the winter sets in.