A “half sider” parakeet

January 26, 2014 • 7:57 am

Here is a parti-colored budgie (“budgerigar”, origin of that name unknown), Melopsittacus undulatus, a native of Australia but of course now found as a pet bird throughout the world. This one is divided is divided transversely in half by color, which makes it very rare—or at least so argues the YouTube video.

How did this come about? The video suggests “tetragametic chimerism“,  the fusion of two distinct zygotes (different eggs fertilized by different sperm).  Apparently this condition is more common in animals than we know, and is also found in marmosets (“most marmosets are chimeras, sharing DNA with their fraternal twins”) and even humans.  It’s not easy to detect unless you do DNA sequencing from different parts of an animal, or the fusion occurs between twins of unlike sex, which can lead to abnormal manifestations of secondary sexual traits (and ambiguous genitalia), or, most readily, when the fused “twins” have different pigmentation, as in this case:

Since birds lay eggs, if this is a chimera it would have resulted from an egg with a double yolk, each yolk representing a different fertilized zygote.

Now I suppose it’s possible that this is not a chimera but a mutation affecting color that occurred at the two-cell stage, but I consider that unlikely given that the color differences are not just the blue/green of the body feathers, but the yellow/white on the head.

The video calls this budgie “one of nature’s most beautiful mistakes,” but reader Su, who sent me the video link, commented “I say not!” The video also notes that “You will probably never see one of these as perfectly divided in any animal group,” but that’s also not true. Remember the “half-sider” cardinal, one of the all-time most linked-to posts on this site?:


We’re  not sure whether that bird is a chimera, a mutation, or a “gynandromorph”, in which one of the sex chromosome got lost at the two-cell stage (see the discussion at the link).

h/t: Su

33 thoughts on “A “half sider” parakeet

  1. Reference the following quote;

    “Now I suppose it’s possible that this is not a chimera but a mutation affecting color that occurred at the two-cell stage, but I consider that unlikely given that the color differences are not just the blue/green of the body feathers, but the yellow/white on the head”

    Is it not possible that this is a mutation from the two cell stage, where the left side (as we see it) merely lost its yellow pigmentation, resulting in a white head and blue body.

    Or am I taking my old paint mixing art lessons to literally within genetics?

      1. Ah red, pigments work differently to paint then haha.

        That would make it half Anerythristic, assuming the terminology works the same as reptiles.

        Maybe even half het Anery too. Would be interesting to know how that would play out in the gametes (assuming this is what has happened)

    1. I agree. Budgie colors are sometimes used in genetics textbooks as an example of modified Mendelian genetics. In budgies, most feathers that make the dark brown/black pigment called eumelanin come out looking blue b/c it reflects most strongly in that color for some reason. Lets just call it the blue pigment. A second gene is needed for yellow pigment. So budgie color is controlled mainly by the combination of blue and yellow pigment producing genes, + various modifier genes that effect the distribution and intensity of a given pigment.
      A common color pattern has both blue and yellow pigments made on the body, producing a green body. A modifier gene keeps blue from being made in the head, so the head is only yellow.
      The bird in the picture could be a heterozygote of the common color pattern, but a mutation took out the yellow pigment in one cell at the two cell stage. So half the bird is yellow everywhere + blue on the body, making a green body with a yellow head. The other half of the bird is descended from the mutant cells. This side can only make blue. Since there is also no yellow in the head on this side, it is white.

        1. Yes, but it turns out that the structures that make this color only work if they are backstopped by an opaque layer of melanin. An albino budgie would be white, not blue.

    1. This is what I found:

      noun: budgerigar; plural noun: budgerigars

      a small gregarious Australian parakeet which is green with a yellow head in the wild. It is popular as a cage bird and has been bred in a variety of colours.

      mid 19th cent.: of Aboriginal origin, perhaps an alteration of Kamilaroi gijirrigaa (also in related languages).

      1. What they did say we could not guess; but by their loud clamour and gestures all the leading men seemed to be in a most violent passion. One word only they knew of the language spoken by our stockmen, and that was budgery, or good; and this I concluded they had learnt at some interview with Dawkins, who used it ever and anon in addressing them.

        From Three Expeditions into the Interior of Eastern Australia [With Descriptions of the Recently Explored Region of Australia Felix and of the Present Colony of New South Wales] pub 1839 by Mitchell, T. L. (Thomas Livingstone) (1792-1855)

        The Dawkins referred to above is “Henry Dawkins, servant to Mr. White”

        This is free on Kindle as a two volume set. Wonderful illustrations of Aboriginals & animals in the PRINT VERSION ~ haven’t downloaded the Kindle yet to see if it has the images.

    2. OED

      Etymology: <Australian Aboriginal language (‘Port Jackson dialect’, Morris Austral English), < budgeri, boodgeri good + gar cockatoo.

      1. Interesting, then, that budgies are not (now) native to Port Jackson (Sydney) and the rest of the south-east coast.

        Possibly the range contracted as changed use of fire plus overgrazing by cattle and sheep led to loss of suitable grassland habitat. Also quite possible that the name was originally used in a broader (not species-level) sense, or else transferred from some other parrot or cockatoo species by Europeans, like the way ‘elk’ was applied to red deer instead of Alces (“moose”) in North America.

  2. Can one breed 2 chimera and get 4 quadrant children and grandchildren that look like checkerboards? 😉

    1. Mark, thanks for those links. I adopt budgies from parrot rescues and have disdained mutation info, but this entire discussion has changed my mind.

  3. This is the budgie for people with kids that cannot make decisions in the pet store. I used to always have a pair of budgies in the house, now I have a Conure. I miss my budgies, I miss quiet time.

  4. In your “half-sider” cardinal post, you said, “I’m not sure whether chromosome loss is involved in the production of this cardinal, or how that loss affects sex determination in birds.” In this budgie, the cere (area above beak with nostrils) unambiguously indicates that he is male; there’s not the slightest bit of tan or brown (female).

    You’ve motivated me to read up on these birds’ reproductive outcomes. 🙂

  5. Hi folks some interesting comments been left here, think I may have a half sider budgie I will try 2 put his picture on soon so would appreciate your opinions thanks!!

  6. Is my budgie a half sider??? His picture is on twitter under @ihamilton (Look for budgie) I would appreciate some guidance thanks

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