Guillemots are birds of Arctic regions. They feed on fish that gather

So poor GMATNinjaTwo lives in New Jersey, where he's suffered through something called a "bomb cyclone" that caused sub-zero (Fahrenheit!) temperatures. He says that he saw a guillemot in his own backyard last week, but that sighting hasn't been verified. And he recommends this two-minute video about guillemot chicks: https://video.nationalgeographic.com/video/guillemots.

Anyway, back to the GMAT... the conclusion of the argument is that the guillemots’ range will probably be enlarged by being extended northward along the coast IF the warming in the Arctic continues. How does the author arrive at this conclusion?

• We are told that guillemots reside in Arctic regions and feed on fish that gather beneath thin sheets of floating ice.
• The birds nest on land near the floating ice.
• The birds need 80 consecutive snow-free days in a year to raise their chicks. Until recently, this fact limited the birds' range to the southernmost Arctic coast.
• However, average temperatures in the Arctic have begun to rise.
• The author thinks that the rise in temperatures will cause the birds' range to be extended northward along the coast. This might make sense... we can infer that it gets colder as you head north, so at some point along the coast there's a limit beyond which it is too cold/snowy for the birds' to raise chicks. If it starts to get warmer in general, that limit might be pushed northward.

Now we need an answer choice that would WEAKEN this logic:

An unusually early snow could happen even along the southernmost coast of the Arctic. As long as this is an unusual occurrence and not a regular occurrence, it would not prevent the guillemot's from moving northward. Sure, it would cause some bad years for the guillemots, but that would be true regardless of where they are living (i.e. regardless of how far north they live). Eliminate (A).

Choice (B) is tempting, but make sure you notice the word "current". The guillemots already have predators in the south. That doesn't PREVENT the guillemots from living in the south; it simply provides a challenge that the guillemots have to deal with.

If there were FEWER predators farther north, that would certainly make it easier for the guillemots. But if the guillemots' current predators simply move north as well, then the guillemots will face the same danger that they did in the south. Again, this wouldn't PREVENT the guillemots from living in the north; instead, it would mean they have to deal with the same predators that they deal with in the south.

Choice (B) can be eliminated.

This fact has no bearing on the author's argument. Will the rising temperatures allow the guillemots to expand their range? Choice (C) provides no evidence either way and can be eliminated.

The point of the argument is that "the guillemots’ range will probably be enlarged." We're not concerned with decreasing the number of guillemots, only the size of their range. The author's reasoning is that their range will expand as the bird moves northward.

The author assumes that the bird will only add to its range as it moves northward. Choice (D) suggests that the northward expansion will come with a decrease in the range along the southern coast (if the thin ice disappears, the fish that gather under the thin ice might have to move northward). Although this doesn't necessarily disprove the author's point, it's seriously weakens the argument. Hang on to this one.

Choice (E) suggests that the guillemots would have LESS competition for food as they move northward. This is evidence that the guillemots WILL be able to survive as they move in that direction. If instead choice (E) said the opposite (that the fish are preyed on by a wider variety of predators farther north), then it would weaken the argument. As is, choice (E) strengthens the argument and can be eliminated.

(D) is the best answer.