Also known as "halfsiders" or tetragametic chimæras, these unusual birds are actually two genetically distinct individuals -- twins -- fused into one being.
I recently stumbled across a video that has been attracting quite a bit of discussion. This video shows a captive-bred pet budgerigar, Melopsittacus undulatus, that is half green and half blue, and the colours are split down the middle. This bird is composed of two distinct individuals fused into one living, breathing being. Yes, you read that correctly. Although this phenomenon is rare, it does pop up often enough amongst captive birds that aviculturists have given these strange birds their own name: "half-sider".
This video is interesting but the narrator's script is quite melodramatic and contains a number of factual errors, which was my impetus to share it with you: