Bird (chicken) development

As far as I'm aware, apoptosis (the programmed death of cells) has nothing to do with the short tails seen in modern birds. Tail vertebrae do form in a developing chicken, but the number that appear is smaller than in dinosaurs such as T. rex - the number of segments in the tail has simply been reduced. Of the tail vertebrae that do appear, a few at the front are incorporated into a structure called the synsacrum, which consists of the pelvic bones and the adjacent vertebrae. (Because all the bones are fused together, the synsacrum looks a bit like a bony carapace in the pelvic region of the chicken skeleton.) The backmost few tail vertebrae also fuse together, to make a bladelike structure called the pygostyle. Apparently the pygostyle is colloquially known as the "parson's nose" or the "pope's nose", although I had never seen either term before I started looking at anatomy textbooks. In any case, there are a half-dozen or so individual, unfused tail vertebrae in between the synsacrum and the pygostyle, but they remain very short throughout growth. Behind the synsacrum, then, an adult chicken does have a free tail consisting of a few short vertebrae and then the fused pygostyle. The bony tail is obvious in a chicken skeleton, but in a living chicken it's obscured by flesh and feathers.