Final answer:
Birds have a fused structure called a pygostyle that supports their tail feathers. It is formed by the fusion of the posterior caudal vertebrae.
Step-by-step explanation:
Birds have a fused structure called a pygostyle that supports their tail feathers. The pygostyle is formed by the fusion of the posterior caudal vertebrae. It plays a crucial role in providing stability and support to the bird's tail feathers during flight.