What is half sider budgie?

Half sider is not genetic, it's a unique defect that occurs accidentally when two different yolks fertilize with two different sperms and then those two separately fertilized eggs fuse together. In birds, this happens exactly just before the eggshell forms around the zygote. This fusion produces a rare kind of budgie that is separated with two colors vertically right from the center, like half blue and half green(classic half sider). 

Half siders are totally opposite of identical twins. Instead of a single fertilized ovum splitting into two embryos, it's two different zygote fusing and resulting in one embryo. This doesn't happen only in budgies but it is also seen in humans, cats, cockatiels and in different kinds of birds. Here are some pictures of half-siders. 

How to produce a half sider budgies? Or How can I get half sider budgie?

Some people think to put two different colors of budgies together (Blue and Green) will result in classic half sider but this is totally wrong because if the green parent is homozygous green, it would have chromosomes for blue in order to obtain blue& green half sider.  Getting half sider is only possible by accident because it’s a combination of two fertilized egg fused together and it has two different sets of DNA, so the half sider budgie can possess organs and body parts that have two different sets of chromosomes. Like half sider by visualization can have one eye controlled by one set of chromosomes while the other is controlled by the other one. So, a half sider could also be a half albino, half lutino or half green. This all depends on the fusion of the eggs. 

What will happen if half sider breeds with another budgie?

Breeding half sider with another budgie depends on the formation of the reproductive tract of half sider, that whether it is formed from single set of chromosomes or from both sets. Like if it is from a single set, the half sider can either create Bb gametes "or" bb gametes (in case of blue & green half sider, this will raise a normal chance for offspring). On the other hand, in the case of both sets, it can create Bb "and" bb gametes and can raise a 3x chance for blue offspring from classic blue and green half sider.  

What will happen if the gender of the zygote is different from each other?

If the gender of one zygote is female and other is male during the formation of half sider, there is a chance that half sider budgie is infertile due to a lot of reasons including malformed genital parts and they might have two color of cere one side blue and another side white or brown. 

In the video below you can see some living examples of half siders budgies.