Until Recently, the Myrtle Warbler and the Audubon's Warbler Were

Until recently, the myrtle warbler and the Audubon's warbler were thought to be separate species of birds because the males have very different appearances. Which of the following observations most likely led to the reclassification of these warblers as one species?

A) The myrtle warbler and the Audubon's warbler have the same diet.
B) The myrtle warbler and the Audubon's warbler lay the same number of eggs.
C) The myrtle warbler and the Audubon's warbler have overlapping geographical ranges.
D) The myrtle warbler and the Audubon's warbler interbreed and produce fertile offspring.