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.
Guide books often establish categories to help birders understand the effort that might be needed to see a particular species or to reward a birder on her search effort (or very good fortune!).
The Princeton Press guide books define frequency with what it calls abundance terms.
Common are birds "encountered (seen or heard) on ostensibly all days" in appropriate range and habitat/season, often in relatively large numbers. Robins are considered common here under those terms (author's opinion).
Fairly common birds are "encountered (seen or heard) on most or all days" in appropriate conditions but in smaller numbers than common species. Broad-winged hawks are fairly common in Minnesota.
Uncommon birds are "not usually encountered (seen or heard) on most days" in appropriate conditions and then in small numbers. This classification also covers birds seen most days but in small numbers because of limited or specialized habitat. Louisiana waterthrush might be considered uncommon here.
Scarce birds are "species that appear rare, perhaps more due to behavior or observer coverage than to actual rarity." Perhaps spruce grouse deserves that label in Minnesota.
Rare are "species that occur in low density, missed far more often than encountered in a day in the field, perhaps only a few times a year." In Minnesota, perhaps goshawks.
Very rare species, in Princeton guide books, are "not usually encountered every year, and should be documented carefully." White-faced ibis might have qualified as very rare here; it is listed by the MOU as casual, meaning not seen every year.
This year, though, and in recent years it has been seen more and more often, particularly in the western part of the state. A flock of 66 was seen on the last weekend of April near the town of Marietta.
That is likely caused by a changing climate that in coming years is going to play havoc with many book and checklist classifications.
Lifelong birder Jim Williams can be reached at email@example.com.