Over the past decade, researchers studying the brains of sparrows have

them is referring to Sound waves (plural).

Shivanibh25

The only way to make IT work would be to have it refer back to "sound control system." But once the message is produced, the system's job is done. It's the messages themselves (or the sounds--they are really the same thing) that the sparrows use to find mates.

VERITAS PREP OFFICIAL SOLUTION:

This problem is filled with pronoun choices, so focus your attention there. At the start of each answer, you have a choice between “their” and “its.” Carefully go back to find the proper antecedent – in this case it is “each sex” so it should be: “each sex uses…its sound control system”. From this error you can eliminate (A), (B), and (C).

In choosing between (D) and (E), there are several diction choices (“to convert” vs” for converting” and “to find” vs. “for finding”) but you should look for a more concrete decision point, such as “it” vs. “them” or “uses” vs. “using”. The pronoun choice of “it” vs. “them” is confusing until you carefully consider the meaning in the sentence: “each sex uses its sound control system to convert sound waves to social messages and then uses the social messages (them) to find mates…” If you missed this and thought that “it” referred back to “sound control system” you can also see that “using” is incorrect. In (E), with the use of the gerund “using” instead of the active verb “uses”, this structure must go back with other gerund “converting” but this meaning is illogical: “each sex uses its sound control system for converting…and then using IT (the sound control system?)”. The sound control system cannot use itself for doing something! Only (D) uses the proper pronouns and creates a logical meaning with the use of a separate active verb “uses.”