So trying to fit in what Mark said with what I saw, the most likely explanation is that the cat injured the Female Blackbird. Because she was injured she couldn’t fly off – the male took this as territorial behaviour, and a threat to his food supply, he probably tried to chase her off and it appeared she was refusing to leave and perhaps was challenging him. I am sure if she could have flown off she would have. [This would explain the first bit I saw where they were flying around each other.] So when she wouldn’t leave he decided to attack her, and in her weakened state it didn’t take much to kill her. Thanks again to Mark Avery for taking the time to explain this and solve the puzzle.