 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 anatomic 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 species (including ruffed grouse, hazel grouse and common snipe) have a number that varies among individuals. Domestic pigeons have a highly variable number, due to centuries of selective breeding.
