*could* happen, but the evidence would seem to indicate that it is still manageable and one is still able to mitigate risk of infection. The fact that the broilers water and feed are not (or should not be) accessible by wild birds is a big help. Risk of infection decreases very quickly once fecal matter touches soil unless the migratory birds have bedded down in big numbers leaving a soild layer of poop - it could be a risk to move the pasture pens over that layer of poop, but that would be the case even if you weren't looking at hpia as a factor.