I second chickbird. Antibiotics have a use, and it is to treat acute infections susceptible to that particular medicine. They are not made for long term usage, and if you have had to use it for four months straight in order to keep your birds healthy you need to seek help from your vet, state vet, or elsewhere as what you are doing is not solving the problem. By dosing with tylan long term, you can be creating antibiotic resistant microbes that could be the source of a more widespread problem. Please, be careful. Antibiotics are a precious resource, and are very vunerable to misuse. Baytril was pulled to vet use only due to concerns over misuse and overuse leading to resistance. R&D money is not going to antibiotics as much as it is to other drugs, and it may be quite awhile before we see antibiotics with different modes of action hit the market.