Interested to find this out too but I'd thought if they were indoor birds and not quite so affected by the changing seasons that they would be fertile. Electric light has some bearing on it so I've read because it prolongs simulated daylight instead of the shorter winter nights in nature.
We recently put her in the cage with her brother, I'm assuming they figured it out naturally. She has never laid an egg before and her inexperience shows when she sitting but not on the egg. Her brother will do the same thing, they have not quite got it right yet. So the egg is fertile then.