In some birds such as albatrosses and falcons, the primaries are molted into two or more non-simultaneous replacement series on each wing (MILLER 1941, LANGSTON & ROHWER 1995, PYLE 2005, EDWARDS & ROHWER 2005), while in other birds the primaries of each wing undergo two or more distinct waves of feather replacement, at the same time and in the same direction (ROHWER & WANG 2010).