Once the nesting season is over for birds, many promptly start a molt of some or all of their feathers. Birds will lose feathers, many of which are worn and tattered from the rigors of raising young, and those feathers will be replaced with new feathers.
While birds are molting, they take on a disheveled look, while old feathers drop and new ones push their way out. But this process is necessary for birds as a well-functioning coat of feathers is critical to a bird's survival.
You may see birds in varying degrees of molt for the next couple months. But don't look for the American Goldfinch molt just yet - they're still busy breeding and all their energies are directed towards that. They'll begin their molt after child-rearing is done and they have energy to put into growing a new coat of feathers.