This is not a simple genetic mutation, as can be observed in other color and pattern variations in this species. It is a rare example of a tetragametic chimera, which originates when two fertilized embryos merge during a very early stage of development — between the 2-cell and the 64-cell stage. Each half has different DNA, with genetically distinct cells and the resultant bird is in effect two budgerigars fused together to form a single autonomous individual.

Sounds like a Steven King novel