I just wanted to put it out there that oregano oil(edible not sure about essential oils which are often just scent) at a dosage of 5 drops per kilo feed, every other day, has resulted in a huge improvement in general health of my visiting pigeons. I mix either oats or barley soaked, with ALL Weather Bird Seed and add 5 drops to the kilo buckets and I have noticed less coughs, no sick or ailing birds and a huge reduction in those one eye infections you seem to get in feral pigeons. In fact I haven't seen any for at least a month now. They seem generally healthy, which wasn't the case in summer but, since giving them fresh water, occasionally with garlic cloves added and the oregano, they seem really well. I haven't noticed any with green poo for ages either. I reckon I am on to a winning preventative.