For its checklist, the American Birding Association assigns a code number for each bird that has been recorded in North America; generally speaking, the higher the code number, the rarer the bird. As defined by the ABA on its website, for code 6 birds "the species is probably or actually extinct or extirpated from the ABA Checklist Area, or all survivors are held in captivity (or releases are not yet naturally re-established)." Basically, unless you're really old or have a time machine, you haven't, nay, CAN'T see any of these birds, at least not alive. The only exceptions are the California Condor - which persists but is not yet self-sustaining in California and Arizona - and the Thick-billed Parrot, which formerly strayed to Arizona but is endangered in its native Mexico.
