On a completely different note about ducks vs. robins (pretending the above flame war didn't happen): I can't say whether average folks would make this connection, but it seems to me that robins sit in trees and don't walk around on the ground very often, whereas ducks tend to hang out on the ground (or in the water). Since robin droppings (a typical disease vector) would work their way to the ground, whereas duck droppings would just stay there, it seems like robins would be more likely infect ducks. Again, I don't KNOW this to be the case, but that's not the point--the point is that this belief (whether or not it's true) immediately popped into my head when reading the question, and therefore would have skewed my own answer.
But I'm also willing to accept that I think about these kinds of things way more than the average person, so my own answer (or prejudice?) may not be at all relevant. :)
On Mexico vs. the US: Here I'm not quite convinced that this is the same phenomenon--though it certainly is an asymmetric comparison, I'm not convinced that it's because one is a more "typical" country, unless I misunderstand the definitions. I'd say that each country has a set of generalized qualities that it's known for: The U.S. may be known for "freedom", "opportunity", "multicultural", whereas when you think of Mexico you think of "poverty", "don't drink the water", "warm beaches." I'm speaking in broad generalities of subject's potential perceptions on both counts; please no one take offense.
When you're comparing the first to the second, (my hypothesis is that) you look for similarities in the first to the generalities in the second, so Mexico->US means you're looking for freedom, opportunity, and multicultural aspects of Mexico, all of which exist to some degree, whereas US->Mexico you're looking for poverty, poor water, and warm beaches in the US (in my brief example), which don't fit nearly as well (though all three exist, of course).
This may be your exact point, but it feels different than the other examples, in that the others refer to the compared object's distance from an archetypal category (100 is a round number, robins are more like "bird" than ducks). Is it that the US is simply familiar because the subjects are here? What happens then when you ask Americans about Mexico vs. Canada or Canada vs. Mexico? I would bet there's a similar asymmetry to the answers, for the reasons I gave. But that's just a guess.