This is easily noted among shorebirds in the fall, as their body plumage is a mix of new and old feathers. The wings and tail on some birds appear old and worn and will be shed later either on the wintering ground or at a safe stopping spot somewhere farther south. Other birds molt their wing and tail feathers before migrating. Immature birds are in bright, fresh plumage. They also molt into a dull winter plumage a month or so later than the adults but do not molt their wing or tail feathers.
Ducks and geese often make molt migrations away from their breeding areas to more secluded or food-rich areas. Male Canada Geese regularly fly northward into Canada to molt in some sheltered marsh or pocket of water. Many ducks wander westward to appear along the coast or into western Oregon interior valleys. They are often seen in small wetlands or on sewage ponds.
Molting birds are often a challenge to birders trying to identify a bird as to species. Often size, shape, or habits are better guides.