I do not understand why this answer is correct. I am failing to see the logic behind it

Melissa, not only should the seabirds saltwater fish consumption be a constant factor in order to get a meaningful measurement for comparison but also the preservation of the mercury levels within the stuffed feathers.
If the mercury level in the stuffed feathers would decrease each decade by 5% and a century later a measurement is taken and compared to the mercury-level of a seabird today and one would draw the conclusion 'the difference is in the amount of mercury in the saltwater fish' it would make no sense.

Niels

I'm still not satisfy with the answer

Audrey, actually Niels (a voluntary contributor) explained it pretty well. I can add a little.

This is a Necessary Assumption question. Think of it this way: if the assumption is not true (by negating it), then the argument cannot be true. In this case, negating E makes it: "The process used to preserve birds in 1880s DECREASED the amount of mercury in the birds' feathers".

Then the argument that "mercury levels in saltwater fish are higher now than they were 100 years ago" cannot be true.

Choice B is the only other tempting one, but the text does not say anything about pollution so you can ignore it.

In the future, please be specific about what you do not understand. Thanks.

Thanks for putting in those terms. Makes it very clear how E is the correct answer.

Nadia, you are welcome.

Why is it implausible to believe that the diet of seabirds has changed over the last 100 years? There are too many factors involved to assume that they have in general stayed the same.