I have been birding quite a while now but I still learn new things about birds - even our yard birds - all the time.
We have a lot of young birds in our yards at present, including Carolina Wren, Northern Cardinal, House Sparrows, Northern Mockingbird, Downy Woodpecker and Red-bellied Woodpecker. So when I saw an American Robin with less than perfect plumage, my first thought was that it was a fledgling. But when I looked more carefully, I realized it was an adult in molt. It seems that Robins enter molt much earlier than our other yard birds.