Did you know that more robins are killed by other robins than die naturally? They are killers and hate competition from other robins - except a male and female at breeding times of course. This is why it is very rare to see two robins in your garden at the same time. However, they don't have visible territory boundaries like our gardens have, so one robin may have part of your garden and another may have a different part. If one crosses its own boundary in full sight of the other, a vicious fight can ensue or one flies off PDQ. I have seen it at first hand in my garden a few times.