 For most birds , moult begins at a certain specific point , called a focus ( plural foci ) , on the wing or tail and proceeds in a sequential manner in one or both directions from there . For example , most passerines have a focus between the innermost primary ( P1 , using the numbering scheme explained above ) and outermost secondary ( S1 ) , and a focus point in the middle of the center pair of rectrices . As passerine moult begins , the two feathers closest to the focus are the first to drop . When replacement feathers reach roughly half of their eventual length , the next feathers in line ( P2 and S2 on the wing , and both R2s on the tail ) are dropped . This pattern of drop and replacement continues until moult reaches either end of the wing or tail . The speed of the moult can vary somewhat within a species . Some passerines that breed in the Arctic , for example , drop many more flight feathers at once ( sometimes becoming briefly flightless ) in order to complete their entire wing moult prior to migrating south , while those same species breeding at lower latitudes undergo a more protracted moult . 
