In truth it was some apparently very angry birds. I had seen mockingbirds in this type of mood before and glanced around.
Aha! Sure enough. It was my friendly neighborhood cat - Blackie Detroit. (Explanation: When he first showed up, frightened and wary of me, I slowly befriended him and
named him Blackie. Later one of our neighbors and I had a conversation about him and she said she called him "Detroit". Hence his strange compound name.)
At any rate, he was just making a slow reconnoiter of our backyard, seemingly unconcerned about the noisy birds who would occasionally dive-bomb him. But I suspect he was more cunning than he appeared.
Eventually, he went under the fence and disappeared. But still I heard the birds in "high dudgeon". I later looked over the fence and found Blackie Detroit casually reclining on another neighbor's picnic table, just waiting for one of those noisy birds to get within claws reach, I suspect.
At this point, he has moved on again, as have the birds. The outcome is yet to be decided.