Cannot tell from your pictures if it's vent gleet or just runny droppings. Food change could be a contributor. Internal parasites also can cause runny droppings. If you can get a fecal float test done, take a mixed sample of fresh droppings from all of them, that would rule that in or out as an issue. As for the bird flu, depends on where you are, you can google "bird flu map 2022" to see where the majority of cases are. If you have a lot of bird flu in your area, then free ranging might be a risk. This link has a decent map that breaks it down between wild birds, commercial poultry, and backyard non commercial poultry (article is dated april) :
https://pbswisconsin.org/news-item/what-the-2022-bird-flu-outbreak-means-for-wisconsin-poultry/
Free ranging always has some amount of risk, you just have to decide how much risk you are ok with. If you put your general location in your profile, people can see where you are and that is often helpful in giving advice, gives a general idea of climate, and what products you might have available.
https://pbswisconsin.org/news-item/what-the-2022-bird-flu-outbreak-means-for-wisconsin-poultry/
Free ranging always has some amount of risk, you just have to decide how much risk you are ok with. If you put your general location in your profile, people can see where you are and that is often helpful in giving advice, gives a general idea of climate, and what products you might have available.
