I describe primary replacement in molt-migrant Painted Buntings (Passerina ciris) captured during molt in northern coastal Sinaloa. Primary replacement is rapid, start dates vary substantially by year, and, especially in females, start and end dates for the molt overlap broadly in some years. The earliest adult females to initiate primary replacement start at the same time as adult males, but many females initiate molt very late, apparently because their mates desert late broods to migrate and molt early. These issues conspire to make traditional estimates of the duration of primary replacement invalid; thus, I calculate molt duration as D = L/G, where L is the summed length of independently grown primary and G is the primaries' growth rate. Adult females replace their primaries in 30.3 (SE = 4.27) days, adult males in 34.3 (SE = 2.99) days. For 54 hatch-year birds that were molting when captured I calculated start dates for the eccentric molt to show, for the first time for any species, that young birds that initiate this partial replacement of primaries later in the season replace fewer feathers. The intensity of the primary molt in adult females, the sexual conflict that results in many females initiating molt much later than males, and recent habitat changes in coastal northwest Mexico, all conspire to suggest that the cause of the decline in the Midwestern breeding population of the Painted Bunting is likely related to recent increases in the mortality of adult females during their annual post-breeding molt.