Myth of The Half Sider

The Myth of the Half-Sider Budgerigar

by Marcel Bühler

During my visits to breeders in the Middle East, I saw the following show budgerigar at Hamad Helal’s place in Bahrain.

It was fantastic, by far the best half-sider budgerigar I have ever seen.

Colour: Cinnamon Grey-Green – Cinnamon Sky Blue Cock.

Genetic Background and Origin

  • Chimerism: Half-sider budgerigars are considered tetragametic chimeras. In this condition, two fertilized eggs (embryos) fuse at a very early stage to form a single organism.
  • In Humans:
    • Twin Chimerism (Fusion): During pregnancy, two separate embryos may fuse early in development, with one embryo absorbing the other. The resulting individual has two sets of DNA, effectively acting as “their own twin.”
    • Tetragametic chimerism: A rare, congenital form of human chimerism occurring when two separate ova are fertilized by two different sperm, and the resulting zygotes merge into one individual.
  • No Inheritance: Since this is a random event during embryonic development and not a classic hereditary trait, offspring of half-sider budgerigars are not necessarily also half-siders.
  • Prerequisite: Both embryos must possess different genetic makeup (phenotypes) for a visible division to occur, usually into blue and green (or yellow/white).

Appearance

  • The division is often sharply defined along the longitudinal axis, with one half displaying the pigment psittacin (unique pigments responsible for the vibrant red, orange, and yellow colours in parrot feathers) and the other not (blue/white).

Breeding

  • Due to their spontaneous development, these birds are rare and cannot be selectively bred.

In conversation with Hamad and Ali, I learned that the half-sider Cock, paired with a Normal Hen, has already produced four chicks. They noticed that three of the chicks were normal coloured and marked, while one young Cinnamon Hen was somehow “different.”

The colour seemed normal, but the markings were very different, somewhat “washed out,” slightly diluted, and somewhat opalescent—completely different from the other chicks in that clutch.

Since Ali Bouresli in Kuwait also had a half-sider chick hatched around the same time—a half-sider Yellowface Blue – Yellowface Grey Hen—they decided to pair the two half-siders together in Bahrain.

Parents of the Pair

Parents of the Cock:
Sire: Grey / Cinnamon
Dam: Goldenface Cinnamon Sky Blue

Parents of the Hen:
Sire: Yellowface Grey
Dam: Cinnamon Grey

This whole situation presents a very exciting opportunity to learn more about half-siders. For me personally, it was the first time in 50 years that I had seen a pairing of two half-siders.

The excitement continues because the pair already had three fertilised eggs in the first round, but unfortunately no chicks hatched. There are currently two more fertilised eggs in the nest box, which should hatch in the next few days.

In my opinion, it cannot be a coincidence that the Cock has already bred a chick with the aforementioned “special” markings.

But as the Greek poet Euripides said: “Time will tell.”

If the experiment with the two half-sider budgerigars proves successful in producing more half-siders, I will post an update including pictures.

Or perhaps there are budgerigar breeders who have already had experience breeding half-sider budgies?