Rectrices

Rectrices (from the Latin for "helmsman"), which help the bird to brake and steer in flight, lie in a single horizontal row on the rear margin of the anatomical tail. Only the central pair are attached (via ligaments) to the tail bones; the remaining rectrices are embedded into the rectricial bulbs, complex structures of fat and muscle that surround those bones. Rectrices are always paired, with a vast majority of species having six pairs. They are absent in grebes and some ratites, and greatly reduced in size in penguins. Many grouse species have more than 12 rectrices; some (including Ruffed Grouse and Hazel Grouse) have a number that varies.