That is exactly the population I am concerned about. Penned adults of the cockyard that require a great deal of effort to be maintained in top form. Much is invested in keeping them in quality feed and clean water. Having other birds (free-ranging juveniles and waterfowl) in area where they can cross contaminate the water and feeding areas in a pen may make for more frequent disease issues even when it involves single birds being infected. Additionally, not only do the free-range birds make so penned birds are more connected to each other, they also make so penned birds are more connected to other environments that initially would not appear to be impacting your pens. Free-ranging birds messing around in pathogen/parasite infested locations can track the undesirables to locations you work hard to keep clean.
Seldom does a bird exhibit clinical signs immediately after infection yet frequently, even before you detect health issue warranting culling, that infected yet asymptomatic bird can be a source of infection for others. What makes things real fun is some individuals never come down with obvious health issues yet shed disease organisms into the environment to be picked up by more vulnerable individuals.
This is part of the biosecurity issue that can always be considered even in your own yard to prevent disease transmission.
The other problem is with some of the "cold-like" conditions that are vectored by mosquitos that fly from pen to pen at night. To get a feel for that, look at your birds next summer about midnight with a flashlight while wearing glasses. You will want to limit habitat for mosquitos after such an observation.
