What is the long feathers on the back of bird form that?

The long feathers on the back of a bird are typically known as "tail feathers" or "rectrices." These feathers play a crucial role in flight by providing stability and control during aerial maneuvers. Additionally, they can serve various purposes in communication, mating displays, and territorial displays, depending on the species. The length and shape of these feathers can vary significantly among different bird species, reflecting adaptations to their environments and behaviors.