 Once they have finished growing , feathers are essentially dead structures . Over time , they become worn and abraded , and need to be replaced . This replacement process is known as moult ( molt in the United States ) . The loss of wing and tail feathers can affect a bird 's ability to fly ( sometimes dramatically ) and in certain families can impair the ability to feed or perform courtship displays . The timing and progression of flight feather moult therefore varies among families . 
