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.