If that is what it is, then he's very rare. It's caused by the egg actually having twins, but one is absorbed and the other takes on both sets of genetics.The half-sider is actually not a true variety. The trait is not genetically inherited. Rather, it is a congenital condition. Visually, this budgie is split vertically, with the appearance and color of two distinct varieties appearing in splotches or sections divided by the vertical center line.
Genetically he is still a blue bird his genetics do not change just the colouring you see.Wow, I didn't know that they can change colour after a moult. Se, I know nothing about genetics haha. It would have been awesome if he was a half sider though.
So Amelia, he would be a green/blue split or something of the sort? Genetics fascinate me.