We have a German Shepherd Dog, and he is great with the chickens. He never chases them, and helps put them in at night.
However, backyard breeding has done a lot to ruin the "working line" German Shepherd Dog. It's really hard to know what you're going to end up with, unless you've got a good breeder. On top of that, all dogs are individuals. Best you can hope for is to be working with your dog's breeding, not against it (i.e. tending dogs will tend, retrievers want to retrieve, etc.).
Many German Shepherd Dogs have very high prey drive; if yours does, you'll need to work at making sure your dog knows what is to be protected, and is not "prey."
If you are planning to get a "pound puppy", you're very possibly giving a good home to a large breed dog that might not get such a chance in life without you. On the other hand, you're taking more of a chance on health and tempermant than if you buy a working line dog from a responsible breeder. Up to you.
There are major health problems that have developed with German Shepherds. They are prone to hip and elbow dysplaysia, pancreatitis, and can develop an enlarged esophagus. If you are buying a dog, your breeder should be able to tell you about those four conditions, and what they are doing to ensure that their dogs don't have these traits. At minimum, they should be xraying the parents (not the young pups) for hip problems, and should have the Orthopedics Foundation for Animals (OFA) certification to back it up. The OFA registration can be checked at:
http://www.offa.org/
A good breeder who is spending money on certification will be happy to show you their certification. I would be highly suspicious of a breeder who downplays the need to cert with this breed, or claims certification but can't be bothered to produce their certificate.
My GSDs are from breeders, and they've been wonderful around our children and other animals. They are generally very suspicious of strangers, however, and our present male GSD will not let anyone he does not know in the yard without putting them up against the fence and doing a "bark and hold." He is not the least bit accomodating to strangers, so a word of caution if this could be an issue for you.
I've also found that every GSD we have ever had has had a thing for bicycles. Something about the speed they go by at seems to make ours think they see a predator, and they are off like a shot. Good thing for fences. Because of this, I would not recommend getting a GSD if you are near a road, and don't have a fence. Fencing is wise for other reasons as well ... GSDs might be perfectly good with your stock, and perfectly willing to consider your neighbor's stock to be prey.
Good luck.