While looking at all the posts this summer, I realized how different the juvenile birds can look from their parents, just as the male and females of some bird breeds look nothing alike. This can really make it difficult to identify them. So that mystery bird at your feeder that doesn't seem to fly well could be someone's offspring trying to fledge.
Here is a picture of a juvenile Cedar Waxwing that may fit the description of a recent 'what bird is this?' post.