Theropods include dinosaurs like the Tyrannosaurus and Velociraptor, but they also ranged in size all the way down to tiny chicken-sized raptors covered in feathers. Theropods and modern birds also both have spongy air-filled bones, wishbones, and feathers and lay eggs and watch over them.

Theropods first appeared during the late Triassic period about 230 million years ago. During the Jurassic, birds evolved from ancient theropods, and today are represented by 9,900 living species.

To make chickens change their gait, the researchers added a tail, which looks surprisingly like a toilet plunger, to their butts. This changed their center of gravity, and made them walk differently.