It is so sad how everyone wants to dismiss something so simple just because they don't know anything about it.
I don't know about ducks and chickens, but in people iodine is absolutely critical for normalizing hormones. For females, lack of iodine can cause breast cancer and also a ton of diseases. Our soils are extremely deficient in iodine, especially areas inland from the coasts. For me personally, I always had cold feet until I started taking iodine. Now my feet are warm because the iodine regulated my metabolism. I also have a general, happy, well being kind of feeling whereas before I was kind of depressed. (By the way, lack of iodine is the main cause for retardation in children - bet you didn't know that little fact!) Also, iodine used to be called the 'Universal Medicine' before the modern age.
So I believe that iodine in their drinking water would have multiple benefits, not just cleaning their drinking water but also regulating their hormones and also adding some much needed iodine in their eggs. (By the way iodine is an antibacterial, antifungal, antivirus, and anticancer agent.)
I'll let you know how it works as I just added it to their (ducks) drinking water tonight for the first time.
PS- It is my opinion that the iodine that was added to the chickens water helped regulate their hormones so they weren't as crabby and less likely to fight. (I know a few women that could use some!!!)
PPS- Please don't automatically dismiss old wives tales immediately without trying them. There is a lot of wisdom out there that is being laughed at that we really need. Remember, people have lived on this planet for thousands of years before modern (expensive) medicine was around and they survived just fine, and were probably healthier, happier and wiser than we are today.