Here's a photo you won't find in any field guide. This is a White-breasted Nuthatch who is molting its feathers, and it's not pretty. It looks a little moth-eaten. End of summer is the time of year when most birds molt just about all their feathers and replace them with new ones. Some birds also molt at other times of year (such as spring, when some who have a non-breeding plumage, molt into breeding plumage).
Just think about it, birds have one set of clothes to withstand heat, cold, migration, breeding, and wear and tear. Feathers are amazing in that they can last through all of this. But now it's time to change those clothes and grow new ones. You can help birds by keeping your feeders clean and full of fresh seed, and clean water in your bird bath, since it takes energy to grow new feathers.
Come fall, you'll see some nice, fresh, beautiful looking birds. I'm looking forward to this nuthatch looking pristine, maybe I'll take its photo again.