Birding checklists and guide books often categorize birds based on the frequency with which they are seen.
In Minnesota, a master checklist is kept by the Minnesota Ornithologists' Union (MOU), essentially the state bird club. It labels birds as regular, casual, accidental, extirpated and extinct.
The state has 313 species designated as regular, 41 as casual, 83 as accidental, two as extirpated (Northern bobwhite and Eskimo curlew), and one in the extinct category (passenger pigeon).
Regular are species for which there are accepted records in nine or 10 of the past 10 years. (Some species are regular but infrequently seen or difficult to see because of limited range.)
Next is casual: species for which there are accepted records in from three to eight years of the past 10.
Accidental species are those for which there are accepted records in one or two years of the past 10.
The MOU has a records committee that considers reports of birds seen. Prior to the ready availability of photos most decisions yea or nay were based on written summaries of the sighting.
Discussions in those meetings were/are rigorous, the organization taking very seriously the distribution of bird species, considering it important science.