This weakens the argument by providing a potential alternative explanation for why cardinals became common in Nova Scotia. Instead of warmer winter temperatures, it might have been the increased availability of food in winter, thanks to more bird feeders being available, that allowed cardinals to proliferate in Nova Scotia.

Irrelevant. This answer choice might be suggesting that northern cardinals are more vulnerable to predators than other birds are. But having this information doesn't help us evaluate whether warmer winters allowed them to proliferate in Nova Scotia or not.

This answer choice is perfectly compatible with the author's conclusion: perhaps warmer winters benefitted the other songbirds as well. We’re trying to weaken the link between warmer winters and increasing cardinal populations.

Irrelevant. For one thing, we have no idea if these field observations were based in Nova Scotia. We're also only interested in the northern cardinal, which is a nonmigratory songbird, so knowing about migratory birds doesn't help us weaken the argument.

Irrelevant. It's possible that the warmer winters allowed these predatory birds to expand their ranges, too, or that they moved north in response to the increasing cardinal population. This answer choice doesn't weaken the claim that warmer winters allowed northern cardinals to thrive in Nova Scotia in the first place.