Moulting is normally symmetrical. Thus a bird may lose primaries and one or two separate secondaries on each wing - when these are replaced they will lose the next ones along. This way a bird can still function. The feathers grow quickly and it requires a lot of energy to build them, even so moulting can take a long time and some parrots take up to 9 months to complete a moult. Some larger birds of prey are an exception to the moulting every year rule and only moult their flight feathers every 2 years.
Many water birds moult all their flight feathers in one go.
This means that for a part of the year they cannot fly. Often they congregate in huge flocks during this time. This is because the larger the group, the less the chances of being eaten by a predator - there is protection in numbers. Moulting costs in terms of resources, a bird's metabolic energy usage may be as much as 30 percent higher during moult. These pages are here free for you to use, I would like to keep them that way, however making and running this site, and the other sites I am currently making or have made, on Birds, Mammals, Insects and Fish and Bacteria costs money.