In older bird guides, the audubon warbler and the myrtle warbler were classified as two separate species. for many decades, the two species lived side by side within their habitat range. in more current bird guides they are classified as eastern and western forms of the same bird species.
since the two warblers are now classified as different forms of one species, they must
a) now occupy the same niche.
b) be identical in appearance.
c) be able to successfully interbreed.
d) consistently live in the same geographic range.