Because most animal muscles are used for both rapid and slow movements, they contain both white and red muscle fibers, as well as hybrid fibers that combine some characteristics of the other two. The proportions of the different fibers in a given muscle depend on the inherited genetic design for that muscle and the actual patterns of muscle use. Frogs and rabbits, which make quick, sporadic movements and use very few of their skeletal muscles continuously, have very pale flesh consisting mainly of white fast fibers, while the cheek muscles of ruminating, perpetually cud-chewing steers are exclusively red slow fibers. Chickens and turkeys fly only when startled, run occasionally, and mostly stand and walk; so their breast muscles consist predominantly of white fibers, while their leg muscles are on average half white fibers, half red. The breast muscles of such migratory birds as ducks and pigeons are predominantly red fibers because they’re designed to help the birds fly for hundreds of miles at a time.