I've never used colloidal silver, but have known people who swear by it. Here on BYC, from time to time someone will recommend it. I'm open to anything that might work to heal a sick or injured chicken, so I hunted for a scientific study on colloidal silver efficacy on bacteria. Found this.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7167925/
Not too many people find scientific papers riveting reading so permit me to summarize. Colloidal silver particles do have certain abilities to affect the cell walls of bacteria, affecting some (gram negative) while being mostly ineffective on others (gram positive), this latter having tough thick walls.
If colloidal silver happens to work on some infections it's because it's a type of bacterium that has weak cell walls. To know what bacteria you are targeting with colloidal silver you would need to have a culture swab identified in a lab ($100) to find out what bacteria your chicken has. Then you might have some success in treating it if it turns out to be a type that is weakened by the silver.
Most of us do not have the means or availability of having a culture done whenever a chicken goes sick, so we want a treatment that has a good chance of working on whatever bacterium happens to be ravaging the chicken. So we select a broad spectrum antibiotic that has a high chance of doing enough damage to either gram positive or gram negative bacteria so that our chicken or duck will recover.