Not to worry. It's just part of the bird's annual postbreeding molt.
All birds replace their feathers periodically, a process that's usually gradual.
It's usually gradual with cardinals, too. This might seem pretty traumatic, but the birds appear to take it in stride. And the feathers grow back well before cold weather arrives. Banner Golden-cheeked Warbler photo copyright 1988 Greg Lasley. All rights reserved.