The one-year-old bird normally lays an egg a day but had not done so for almost 48 hours and was looking listless.
Kasha said: "When I saw the size of the egg I thought I must be dreaming. I could not believe it was real when I opened the hatch and looked in. It was twice the size of Gertie's normal eggs and I did not believe it was real until I picked it up.
"Gertie had not been very well the day she laid it and I don't think she will want to lay another egg that size again."