I am a chemist, and have looked into "colloidal silver" products in the past. I am also a person who likes "natural" remedies.
I believe that colloidal silver claims are true, in general. The chemistry/science behind how it works make sense to me. Silver does kill bacteria. But the problem I have with colloidal silver products is the nature/size of the colloid particle is pretty uncontrollable.
The particles have to be very small to get into a virus. Larger particles CAN get into a bacterium. However, without ever knowing what I was getting, I would be leery about depending on this sort of thing. The only Quality Control that is measured is the total amount of silver in ppm - Parts Per Million. You have no way of knowing if you have "jillions of little tiny colloid particles" or "ONE BIG PARTICLE". (OK, that's an exaggeration, but you get the drift).
I have found some IONIC silver products that I have had fantastic personal results with. They are made by Peaceful Mountain, the line is Sinus Rescue, Throat Rescue, Stomach Rescue, and Cold Rescue. Sinus and Cold are the same except for dosage - Sinus is 30 ppm and Cold is 20 ppm. I think Throat is 40 ppm and it has some mint in it - it's a throat spray. Can't remember the Stomach one, never really needed to use that one. I used to fly a lot - if I used the Cold or Sinus nasal sprays on my trip, I would not get sick when I got home.
If I forgot them or didn't use them, I would stand about a 40-50% chance of getting sick after a plane trip.
The key with Ionic instead of colloidal products is that the INDIVIDUAL silver ions are available. Individual ions = small particles = capability to enter a virus - PERIOD. There is no question about how many might be clustered together, etc. and whether you are actually getting the benefit of the silver that is there. It's all "bioavailable"/active. So when you have 30 ppm ionic silver, you are GETTING 30 ppm silver. When you have 30 ppm colloidal silver, your viruses might not be seeing anything at all depending on particle size...
Hope my Chem 101 description made some sense.
The lab I used to work at is closing down. I know they have a small bottle of silver nitrate in the cabinet - I'm hoping I can mooch it from them before they turn out the lights...
Just realized I didn't answer the "original" question about it being SAFE. DISCLAIMER: I am not a vet, and I've never given silver products to a chicken, but I can't imagine anything about it that would be inherently UNSAFE. It's a "simple" thing, a basic metal/mineral, not like some of those complex interactions that can occur with different species (dogs can't eat chocolate or raisins, these things can kill them!) or with some poisonous plants. I would THINK it would be safe.