Odd bedfellows

For the past three years we have oystercatchers and herring gulls nesting on our roof. Last year I emailed the RSPB and asked them if the birds were doing this for mutual security. The reply came, no gulls will predate on other chicks. If that was the case, why do the oystercatchers keep coming back? It’s been fairly calm so far this year apart from one immature gull chick falling to the ground. My new neighbour was trying to defend himself with a rolled up brolly as the angry gull dived. Better to get out of the way I thought.
    It’s impossible to get pictures of what’s going on up there without a drone so these are from three years ago. This adult was tapping on our glass of our main door, thinking that the downstairs lobby looked like a safe place to hide out wth it’s chick. Extra of oystercatcher and chick.