It is that time of the year again, babies are hatching, parents are feeding and the chicks are starting to feather up. It is my favourite time of year as a budgie fancier – it is the time when all of our hard work is, hopefully, rewarded with beautiful young birds on the perch and parents who are raring to go again. It is also the time of year when we notice interesting looking birds in our nest boxes. This article will touch on two of those genetic anomalies – the “feather duster” budgerigar and the half-sider budgerigar.

In order to understand the half sider anomaly I will need to use some scientific jargon so please forgive me and feel free to contact me if there is anything that is unclear!

A half sider is a tetragametic chimera; ‘tetra’ meaning 4, ‘gametic’ meaning gametes (sperm and ova) and ‘chimera’ meaning a single organism composed of two different populations ofgenetically distinctcells that came from two different zygotes (early stages of fertilized cells). In order to form, two fertilized eggs must fuse together.

Put simply, half sider budgies are formed when two fertile yolks come together to create one egg.

It happens purely by accident and is not genetic. It occurs when the female has released two yolks from her ovary at the same time (not an uncommon occurrence) and they fuse, after each is separatelyfertilized by different sperm.

Because the half sider is essentially the combination of two baby budgies fused together, it has two different sets of DNA. In a visual half sider, pigment on one side/part of the body is controlled by one set of chromosomes while pigment on the other side/part of the body is controlled by the other set of chromosomes from the other zygote, which is why half siders are generally two colours. If the zygotes had not fused, you would have gotten two healthy, perfectly normal budgies (one blue one and one green one). Half siders have the potential to be all combinations of colours, it all depends on the parents and their genetics!

It is for this reason that the half sider budgerigar is not a real mutation – by breeding two half sider budgerigars together, you will only create another half sider by chance (the exact same chance that created the initial budgies). When it comes to breeding a half sider with another budgie it all depends which of the budgie’s chromosomes formed the reproductive tract of the half sider was formed. A very important point to consider is that if the gender of one zygote was female and the gender of the other was male when the half sider was formed, it is highly likely that your half sider budgie is infertile due to malformation of the reproductive tract hermaphroditism. Hermaphrodite half siders may have two colours to their cere, one side blue, the other side brown.


Article provided to the NZBS by:

Dr Hamish Baron
BVSc (Hons) MANZCVS (Avian Health)
Resident | Avian Reptile and Exotic Pet Hospital
Faculty of Veterinary Science