Bird guides once listed the myrtle warbler and Audubon's warbler as distinct species because of differences in color patterns of the feathers and size of the beak. Recently these birds have been reclassified as eastern and western forms of a single species, the yellow-rumped warbler, based on the observation that the two forms interbreed often in nature and the offspring have good survival and reproduction. The earlier classification into two species was using a species concept; the reclassification is using the species concept. _________