Half-sider budgerigar

This is not a simple genetic mutation, as can be observed in other color and pattern variations in this species.

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.

[1] The half-sider's coloring is usually divided bilaterally down the center, although, it can differ depending on which stage the twin embryos merged during development.

Twin embryos that merged later in development will result in a budgerigar that has a splotchier distribution of the different cell populations.

[1] In the case of the half-sider budgerigar, both embryos must possess different genetic phenotypes (one yellow-based and one white-based)[2] in order for a visible half-sider to be produced.