Past studies of the replacement rules for flight feathers of cuckoos have been plagued by the erroneous assumption that the primaries constituted a single molt series. Instead, we show that the primaries of the Yellow-billed Cuckoo (Coccyzus americanus) are divided into three molt series and those of the Common Cuckoo (Cuculus canorus) into four series and, further, that Yellow-billed Cuckoos replace their rectrices in two molt series. Multiple molt series make it impossible to consistently describe primary replacement as a unique sequence of feather loss, as earlier researchers have attempted to do for cuckoos. We calculated the mean number of days (± SE) required to replace the primaries in each molt series. Primary replacement takes considerably less time in the larger Common Cuckoo than in the smaller Yellow-billed Cuckoo, mostly because Common Cuckoos replace four primaries at a time whereas Yellow-billed Cuckoos replace only three at a time. The primaries of Common Cuckoos cannot be better allocated into four molt series such that the time required for their replacement would be reduced, but molt duration would be reduced in Yellow-billed Cuckoos if their primaries were differently allocated into three replacement series. Both species replace their primaries faster than birds of similar size whose primaries are organized in a single replacement series.