Two distinct forms of Red Crossbill (Loxia curvirostra) bred sympatrically in the southern Appalachians in 1983. The forms differed significantly in vocalizations and bill and body size, and the members of each of 20 male-female pairs were of the same size class. One of the forms from Virginia has been tape recorded in western North America in 1983 and in previous years, showing that vocal classes are not local dialects. Evidence presented here suggests that forms of Red Crossbill are not subspecies as is currently recognized, and that they behave as biological sibling species. / Master of Science