North American Birds: ABA Code 4 - Page 3

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, code 4 birds are "species not recorded annually in the ABA Checklist Area, but with six or more total records—including three or more in the past 30 years—reflecting some pattern of occurrence."