Most birds molt their flight feathers in a fairly straightforward way, as explained in the last blog. The feathers on each wing molt sequentially, starting with the innermost primary and usually ending with the innermost secondary. This is called sequential wing molt.
But there are two other ways to do it.
Very large birds (above about 1 kilogram in weight) that need to be able to fly at all times, for example eagles, cormorants, and herons, can retain feathers for more than one year and molt only some of them each year. The largest among these take about three years to replace all the flight feathers. This molt strategy is called stepwise wing molt.
In many large birds, you can see the different feather generations in an outspread wing because the older feathers are more worn and thus a bit paler. These differences should be evident in this Turkey Vulture wing specimen from the Slater Museum collection.
The other strategy is simultaneous wing molt. If predation is the only reason they need to be able to fly, and birds can avoid predation, they can give up flight for a period of time and drop all their flight feathers simultaneously. This is the case in anhingas, ducks and geese, loons, grebes, and larger alcids, all of which can remain in the water during this period, out of range of their potential predators. Note this group includes the very large swans and geese, which perhaps could not fly very efficiently with gaps in their wings.
The female Mallard is typical of midsummer ducks with all flight feathers missing. The Common Murre also is in full wing molt, usually obvious because the wingtips can’t be seen above the tail.
This Anhinga has dropped all of its flight feathers and most of its tail feathers simultaneously and is in the process of growing them back.