If a bird loses a wing feather it is subject to slight aerodynamic imbalances. Yet all birds molt their feathers. So what happens when they molt old feathers? Do they fly erratically during the period of molt?
No. Evolution has developed a system so that when a bird molts a feather from its right wing, the same feather on left wing also molts. Thus symmetrical balance is maintained.
Waterfowl are an exception to this symmetrical molt. Mallard ducks for example do not molt their flight feathers one at a time. They molt them all simultaneously. Without flight feathers they cannot fly. During this period Mallards are in a state called eclipse plumage. It takes a week or so for new flight feathers to grow in during which time Mallards are grounded (or more likely water-bound) and cannot fly at all.
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