The information in this checklist is derived from "Checklist of the Birds of Ohio", produced by the Ohio Department of Natural Resources in collaboration with the OOS Ohio Bird Records Committee. The full 40-page checklist, which contains considerably more information than just the frequency codes, can be found in PDF format on this website, here.
Explanation of Frequency Codes
The codes indicate abundances for each species at their optimal locations within the state. This means that an any one location, many species may be much less common than indicated, or even absent.
C - Common: to be expected, often in large numbers.
U - Uncommon: observed infrequently and usually in small numbers, even in appropriate habitat. This designation indicates that although there is a reasonable chance to detect the species, it should not usually be expected.
R - Rare: normally occurs annually, but with only a few records on average.