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Flight feather molt is a time-demanding activity in the avian annual cycle. Yet, annual or alternate-year replacement of flight feathers is essential, because physical abrasion and ultraviolet light rapidly degrade even the most sturdy wing quills after two years of use. Because flight performance declines during molt as new feathers are growing, most birds do not overlap molt and breeding, and those that do overlap these activities replace few flight feathers at a time, presumably to minimize the energetic and flight-performance costs of molting on reproduction.
-Rohwer, Sievert, et al. “Allometry of the Duration of Flight Feather Molt in Birds.” PLoS Biology June 2009.