Okay, finally have a little time to check in.
For human immune systems, I think the value of walking and dancing are undervalued. Muscle movement pumps the lymph system - and helps rid the body of toxins and pathogens. It also keeps the blood oxygen higher, and core temperature higher - both of those discourage pathogens. With ducks, it's similar, probably - they need their exercise, too.
Lots of green vegetables, and colorful vegetables, some mushrooms, and seaweed. Again, probably goes for all of us, ducks and humans.
I have been trying cat's claw herb. Some folks aren't big fans of herbs, but that's their approach. A couple of others that have some possible benefit, I have read, are astragalus resveritrol.
Glad Kathy is around to help with the tub feeding - I have not had to do that yet.
Hugs to you all.
And yes, picking up my Runners to get a feel for their body condition is the way I do it. I do not do it every month, I am especially likely to check on a broody, or someone who has been under the weather, but periodically I do the whole flock. It's kind of fun, even for them. But I have poured many hours into socializing them.