This went on for some half an hour and then, apart from the irate willy wagtail who was still venting his anger on the back fence, the noise muted to the normal sounds of birds going about their daily business. Obviously there had been some kind of threat but what it was I have no idea. An hour or so later and the magpies were having their usual morning skirmish with the ravens so why they hadn't both been involved when the smaller birds were so distressed remains a mystery, too.
I went to the park later to see if I could see anything amiss but it was calm and peaceful, with not so much as a leaf out of place. Strange.