Males of eastern and western races of Yellow-rumped Warblers look radically different. Eastern birds, called Myrtle Warblers, breed in the forests of Alaska east across Canada into New England, New York, and the Appalachians. Western birds breed in the Pacific Northwest and adjacent Canada, south through the Rockies to our Southwest. They breed east to the Black Hills of South Dakota. In these photos, the top is of an Audubon's Warbler at the Kate Sessions Park in San Diego; the bottom is a Myrtle Warbler in Pierre, South Dakota
Despite their differences, the two races hybridize in the southern Canadian Rockies. In fact, they both are very similar aside from the plumage of the males--behavior, habitat, and song are all similar. The area where they interbreed is very narrow--less than 200 km. One theory is that the two species came into contact as forests grew higher into mountain passes after the last glacial retreat some 10,000 years ago (Hunt and Flaspohler 1998). These authors cryptically comment that the narrowness of the hybrid zone may warrant further study of just how Audubon's and Myrtle warblers are related.