North American Birds: ABA Code 2 - Page 5

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, codes 1 and 2 "include regular breeding species and visitors. There is no firm designation between Code-1 and Code-2 species, except that logically Code-1 species are more widespread and are usually more numerous. Code-2 species have a restricted North American range, are more widespread, but occur in lower densities, or are quite secretive making their detection often difficult. We readily acknowledge that some Code-2 species are harder to find than some species that have higher codes."