That's because most birds are molting. Every bird goes through a complete molt once a year. Most birds molt in late summer and early fall, but the way a bird molts differs among species. Some have a partial molt, then migrate. But they’re not normally in a full molt when flying long distances.
Molting requires a lot of energy, so that's why it occurs after the breeding cycle is over. Birds tend to keep a low profile when molting because they are more vulnerable to predators during this time.
Birds in molt can look scruffy or unusual with stubby tails and patchy bald spots, which makes bird identification tricky this time of year.