Sorry, I edited my link to a USA link from a vet after you quoted me. I still agree with that statement, free range birds do have a higher probability for parasites because they are eating earthworms and other insects that have parasites. However, ALL animals can end up with parasites and most animals (including humans) have small levels of parasites in our systems. It is when they are overcrowded or living in their own feces or generally unhealthy (under weight, sick with infection, etc) that the parasite load can become unmanageable.
My *personal* feelings are that free range birds are healthier than birds that are constantly confined simply by reason of: varied diet, sunshine, exercise, mental health, etc.
I would rather treat my birds for parasites when necessary rather than confine them at all times in the hopes of preventing parasites, that they might still get.
I want to clarify my definition of free range. My ducks do not run entirely free; they are contained within my LARGE fenced, grassed garden/yard and that is separated into sections so I can control which section of the yard they are in if I need to. If one area gets a bit too over used I can keep them out until it has a chance to recover. They are cooped at night and let out early in the morning. We raised all our chickens and ducks like this on the farm I grew up on and we always had healthy birds so I feel comfortable continuing to do this.
My definition of confined is birds always kept inside a coop or small pen.
As always, this is just IMHO so take what you want from it and leave what you don't. These 4 ducks are new to me, as is keeping ducks in a semi-urban environment but I grew up with many ducks, geese and chickens on the farm. I don't profess to be any sort of expert or have any knowledge beyond my own personal experience which is why I like hearing what works for other people. I'll try anything to keep my animals as healthy as possible, as long as the cure is not worse than the disease.