I wanted to start this thread, so here we go. I’ll start:
bicknell’s thrush and gray cheeked thrush should be one species (gray-cheeked thrush).
black capped and Carolina chickadees should also be one species (American Chickadee)
Northern flicker should be two species (yellow shafted and red shafted flicker)
dark-eyed junco should be separated into five different species (slate-colored junco, Oregon junco, white winged junco, pink sided junco, and gray headed junco.
Can you think of anymore?