The Oystercatcher and the Herring Gull

When I was taking this picture a young guy came up to me and asked if these were rare birds on the roof.
“No, it’s an oystercatcher and a gull. They are both nesting up there.”
“So what do they each get out of it?” he asked.
“Security,” I said, feeling confident. “The oystercatcher is a good early warning system, and can make a lot of noise. The gull is more threatening to intruders, so it protects the other bird. Although I’m no expert,” I hastily added.
“So, it’s a symbiotic relationship.” he said. Despite not being entirely sure what that word meant, I agreed that it was. I told him that I lived in this block of flats and the oystercatcher has alreadly lost a chick which fell off the roof. He asked if the mother tried to pick it up.
“No,” I said, “either they can’t do it or they don’t have the thought.”

A short while later I got a brief glimpse of a camera shy oystercatcher chick. I haven’t seen any gull chicks. Sure enough the warning cries went off from both birds as I approached the front door. This is the third year of the current arrangement.

PS Its a shame you can't see the oystercatcher's orange bill, as her head was turned round. A better photographer would have been more patient.