Bird guides once listed the myrtle warbler and Audubon's warbler as distinct species that lived side by side in parts of their ranges. However, recent books describe them as the eastern and western forms of a single species, the yellow-rumped warbler. Apparently, the two kinds of warblers ...

a. live in the same areas

b. successfully interbreed

c. are almost identical in appearance

d. are merging to form a single species

b. successfully interbreed

Explanation:

At first, these two kinds of warblers are described as two different species because of their ranges. However, if they are described now as a single species, that means that they can mate between them. A species, in general terms, is defined as a reproductive unit of organisms, so if a group of organism can produce fertile progeny, it is called "species".

Living in the same area, or having an identical appearance are not criteria to call a group of individuals "species". So, options a and c are wrong.

Option "d. are merging to form a single species" implies than before, the two types of warbles were different species, and now they formed a new single species. This is not what the rubric says. In this example, the two warbles always have been a single species, but before they were wrongly categorized, probably because it wasn't known that they were able to interbreed.