Unlike the brown sparrow, the passenger pigeon (was

I think a more important factor is to understand what the sentence itself is trying to say. In my opinion, I can't really distinguish exactly what this sentence is conveying to us. Reason being:

1. Is it trying to indicate to us a sequence of events? That the passenger pigeon had been slaughtered, THEN became instinct? Because if the sentence's original meaning is to convey this specific sequence of events, the past perfect here would be the better choice because it indicated that the passenger pigeon FIRST got slaughtered, SECOND became instinct.

2. Or is the sentence trying to simply compare and contrast the mere difference of the extinction of passenger pigeon from that of the brown sparrow? If it's simply trying to tell us that the brown sparrow did not go extinct, but the passenger pigeon did, I believe simple past tense would fit perfectly well here...that the passenger pigeon was slaughtered and went extinct.

That's my take on this sentence; I'm not sure if my reason behind the ambiguity makes sense. If it were me taking the test and I encountered this question, I believe I'd choose simple past. The reason is, if the question were trying to point out a sequence of events, the question shouldn't even begin the sentence with "Unlike brown sparrow". The mere fact that this sentence began with "unlike brown sparrow" hints me that the original intent of this sentence is to make a simple comparison between brown sparrow and passenger pigeon. Since the main point is to compare the difference between the end result of brown sparrow and passenger pigeon, I'd go with just simple past - clear and direct.

Sorry for the second post:

Sacmanitin, the best way to understand past perfect is to know that within ONE sentence completion question, there has to be TWO different events that happened in the past - remember that! And because you want to point out the very difference between these two past events, you'd want to use past perfect.

Without two different past events, you do not use past perfect, but rather you'd use just simple past.

The construct of past perfect is the usage of the verb tense "had". The event that happened first should have the verb "had", as indicated by gmatwarrior's excellent example.

Thanks gmatwarrior and bakfed for your detail explanation,
i would like to share the oficial reasoning as well so that we all can atleast relate and correct and do some more brainstroming ,it says :The simple past is best here because the sentence describes an action in the past that has been completed.

simple past works fine here since it makes no sense to talk about becoming extinct BEFORE being slaughtered...

obviously "was slaughtered" happened before "becoming extinct" so we do not need to convey one happened before the other using "had + past participle"

Simple past --

Unlike the brown sparrow, the passenger pigeon was slaughtered indiscriminately and became extinct in 1914.