Is My Budgie Egg Fertile Or Not?

Hi all, my budgie layed an egg yesterday. It was teh first 1 that she has ever layed. My cage is outside and the box has nothing in it just the wooden floor. Last night she sat on it for most of the night. However this morning she didnt sit on it as much as last night. This morning I took it out of the box and it was cold. I got a torch and shone it on the egg in a dark place.  Near the top of the egg it was yellow. Is this egg fertile or not??