Rectrices

Rectrices 

in birds, the feathers of the tail used during flight. The tail increases the area providing lift and serves as a “rudder” when the bird changes direction. The rectrices are attached symmetrically to the pygostyle (the terminal bone in the tail). They vary in number from four pairs (American ani) to 12 pairs (pelicans, some pheasants).

Rectrices are similar in structure to flight feathers and, like them, overlap one another. The base of the rectrices is covered by the upper and lower coverts. The shape of the tail is determined by the length of the rectrices. In woodpeckers and tree creepers the rectrices are narrow and rigid, serving as a support when the birds climb. The rectrices of peacocks are concealed under powerfully developed upper tail coverts with ocellated spots. Some penguins, storks, and grebes have underdeveloped rectrices.