http://www.birdnature.com/redbird.html

Recently the Baltimore Oriole was combined with the western Bullock's Oriole as a single species, the Northern Oriole. When trees were planted on the Great Plains, the two forms extended their ranges and met. Despite the differences in their appearance, it was found that they interbred, and that most birds in the central plains were hybrids, so the birds were combined into a single species. Now, it seems that in some places the birds are choosing mates of their own type, and they are considered separate species again.

description 7-8 1/2" (18-22 cm). Male has black head, back, wings, and tail; orange breast, rump, and shoulder patch. Female olive-brown, with dull yellow-orange underparts and 2 dull white wing bars.