That's interesting. I think ducks are generally the big vectors in the bird world, but neither the US nor Mexico is a big duck-raising place (unlike China, where duck is a popular food).
I suppose it could have been brought by migratory birds from some place in Latin America, not necessarily Mexico. In other words, because Mexico is more urban and probably has a better health system and more educated population, they were able to notice a surge in cases; other poorer countries in Latin America (where birds winter) may not have even noticed it, especially if it started in a rural area where the population is somewhat dispersed.
It's possible that it's terrorism. But I don't think terrorists would have set this stuff off in any place they are located, unless, of course, they were certain that they had a vaccine. In that case, it would have to be a technologically sophisticated power behind it.
Flyways may be found at: http://www.birdnature.com/flyways.html