In addition, in the long run, having access to the wild population, will make your flock stronger and have a better immune system.if I worried about this I would sell my birds. That is something that just can't be stopped. Even in my breeding pens crows will hang out outside the fence and doves and songbirds go in to eat and hang out.
I tend to not pay attention to much in print. With just all of the WRONG info out there from hatcheries about Sumatra's, I mean REALLY wrong information.... I just make a mental note of things I read and just monitor my flock. That really is the main key to raising birds. Monitor.
I have around 100 birds. So I can't even imaging being concerned about song birds and such. My .02 anyway and for 9.5 years it has worked fine