Well the other thing we have to look at is what are there eating habits like in the wild?
Look at the domestic goat, they mostly graze on pasture with lots of vegetation close to the ground where the "wild" goats can forage more on trees and shrubs. The wild goats are less likely to pick up parasites.
Now the same can be said with chickens, if your pasture/runs are mostly dry dirt with a few shrubs and "off the ground plants" then your birds are going to be less likely to get parasites than birds that are on a pasture/run that is damp with lost of vegetation that grows on or near the ground.
Chris
When we co-graze goats with cattle or sheep we call goats browsers the other two grazers. The assumption is the goats are consuming broadleaf plants while the other two are consuming grasses. Most pastures are not effectively managed to have good balance forcing goats to eat the same things sheep and cattle are consuming.
Chickens in free range settings tend to concentrate their foraging around feeders. This causes animals for pick around in areas that also have higher concentrations of their own feces. I am trying to manage the forage plant community and spread foraging presssure by moving feeding stations around. Problem you get into is that more land is required and daytime predator management becomes more important. Also, some chicken breeds do not get out and away from feeders and thus are inherintly inclined to re-infect themselves with parasites. I am getting frustrated with all the talk about American Dominiques being good forages but they are clearly not when compared to my games. Some of the games ability to keep putting on mass even without supplmental feeding is not only because of better foraging but also because they may not be re-infecting themselves as much with their own worm burden.