There are a number of equally serious errors in this sentence as written. It contains an incorrect usage of the idiom "mistook X for Y"—the preposition "for" should replace "as" in the original. It also contains an illogical comparison, drawing a parallel between the "calling" of the mockingbird and the "rare rufous-sided towhee." Logically, one birdsong should be compared to another, not the song of the mockingbird to the species of the rufous-sided towhee.
In addition, the sentence should use the noun form "call," rather than the verb "calling," to describe birdsong.
Scan and Group the Answer Choices:
(B) and (D) correctly compare the two bird calls. (A), (C), and (E) leave the comparison unchanged from the original. (B) and (C) use "call" instead of "calling." (B) and (D) use the correct idiom "mistook ... for" instead of "mistook ... as."
Eliminate Wrong Answer Choices:
Only (B) and (D) correctly implement the "mistook X for Y" idiom, and only (B) uses the correct noun form of "call." (B) fits perfectly into the original sentence and is correct.
TAKEAWAY: Comparisons must be in parallel form and compare logically identical items.