In an ideal situation, the basics of bird dog training should start around 7 to 8 weeks old. These first steps are primarily used to establish obedience, bonding, proper relationship roles, as well as introduce basics commands. I've never thought an introduction to live birds was a good idea until a foundation of obedience was established. Working now on basic obedience, especially after your pup has learned that chasing and killing birds is fun (chasing chickens does not necessarily indicate a dog is going to be much of duck dog) and likely developed a taste for blood, is kind of like shutting the barn door after the cow has gotten out.
I wish I could be more optimistic on your chances of "turning" your pup "into a bird dog," but it's a pretty tall order at this point. Try some of the suggestions offered here if you like, but I'm inclined to think Crapshoot is offering the most common sense sollution at this point. I guess it would depend on how many chickens your dog has killed and your dog's individual temperment. I'm not exactly a dog training expert, but I wouldn't expend a lot of time and resources on trying to get a chessie/lab mix with a taste for killing birds to be a decent duck dog. I might be tempted to take the dog to the pound, wait a while, read a book or two on proper bird dog and/or retriever training techniques, decide what breed best fits my needs, get a 7 week old pup and start over. This time keeping the chickens and your pup from being intimately aquainted for a while.