This is the time of year when young birds (juveniles) start to molt into their adult-like plumage. If you look closely, you may see a few birds that you recognize at your feeders that just don't look quite right.
It is easiest to see in the blackbird family, especially in Common Grackles, Red-winged Blackbirds, and Brown-headed Cowbirds. In blackbirds, the older feathers of young birds will be brown, while the newer adult feathers will be black. Red-wing males are especially interesting as they obtain their characteristic red-wing epaulette and the wing coverts will be jet black, but their flight feathers may be both black and brown for couple of weeks.
Watch out for this at your feeders to see if you can spot any two-toned birds!