This fourth supplement after the 6th edition (1983) of the AOU "Check-list of North American Birds" consists of changes adopted by the Committee on Classification and Nomenclature between 1 March 1989 and 1 March 1991. The changes fall into eight categories: (1) five species (Ixobrychus sinensis, Porphyrula flavirostris, Sterna bergii, Streptopelia orientalis, and Ficedula narcissina) are added to the main list because of new distributional information; (2) six species (Pterodroma cervicalis, Ortalis wagleri, Lophornis brachylopha, Corvus sinaloae, Cinclocerthia gutturalis, and Loxops caeruleirostris) are added to the list because of the splitting of species previously in the list; (3) one extinct species (Dysmorodrepanis munroi) is added to the list because of re-identification of the unique type; (4) one scientific name (Speotyto cunicularia) is changed because of generic splitting; (5) one scientific name (Phalacrocorax brasilianus) is changed for nomenclature reasons, accompanied by a change in English name; (6) the spelling of one scientific name (Neocrex colombianus) is corrected; (7) to other English names are changed or corrected; and (8) one sequencing change is made. No new distributional information is included except as noted above (i.e. minor changes of distribution of distributional records within North America are not included). The twelve additions bring the number of species recognized as occurring within the Check-list area (main list) to 1957.