Herring gull with crane egg

The Manchurian cranes (red-crowned cranes) at Paignton live in the area around the howler monkey and pileated gibbon islands. They are a related pair, but they have nested and laid one egg (at least), last year and this year. I don't know why they did not raise a chick last year, but yesterday it appeared that their nest had been abandoned and the egg was lying about a metre away. It is possible that the egg was infertile or that the keepers had replaced it with a dummy egg. It was being studied by three of Paignton's herring gulls (notorious for their fondness for visitors' sandwiches, ice creams etc) and I suspect that they had been moved it from the nest. One gull did manage to lift it up briefly, but it was just too big for them deal with.
My photos show that the egg appears to be real and that its surface has been damaged in places; but I don't know whether this was caused by the cranes, the gulls or by the keepers, to prevent the pair from breeding.