All birds molt - shedding their old, weathered feathers with new ones. Some birds molt once per year, others two or three times per year. Sometimes the molt process is simultaneous, such as with waterfowl when they lose all of the primary feathers at once, leaving them flightless for 3-4 weeks. You’ve probably seen them this time of year in a wetland doing a breast-stroke swim to escape to cover.
Others such as black terns have a very complex molt cycle with it beginning as early as June when their eggs are hatching. These birds can’t afford to undergo a flightless period, so molting is spread out over a longer time and their appearance changes slowly but dramatically.