You don't want a HERDING breed, you want a GUARDIAN breed, there's a huge difference.
Herding breeds (Collies, Shepherds, etc. - Herding Group in the AKC) are meant to drive stock from one place to another, working in tandem with the stockdrover. Leave a herding dog alone with the stock, and the herding dog may harry the stock to death by constantly moving them from place to place. It's what they do, and what they're bred for.
Guarding breeds (Great Pyr, Anatolian, Akbash, Maremma, Kuvasz, Komondor - they're classified as Working Group in the AKC) are meant to live with the stock and protect it from predators. They have been bred for this for thousands of years in some cases, and the "prey drive" instinct that so many other breeds have is severely controlled in these breeds.
http://www.lgd.org
Here is a specific article about Livestock Guard Dogs (LGDs) and Poultry:
http://www.anatoliandog.org/poultry.htm
If you go with an LGD, a couple of words:
1) Get in touch with someone who owns an LGD (particularly a poultry LGD) for tips and information. It's not like owning a pet dog, and there are certain techniques used to acclimate the dog to the stock - and vice-versa.
2) Consider contacting an LGD rescue group to see if they have any working dogs who need a working home who would work with poultry. That may not be an option because none are available, but with the number of small farms going under and the number of working LGDs needing homes, this is always my first recommendation - working LGDs usually don't transition well to pet homes.
3) If you go to a breeder, go to a REPUTABLE breeder of WORKING LGDs, and get recommendations from other LGD owners about where to go. If possible find someone who breeds LGDs and trains them to work with poultry already, so the pups will have some foreknowledge in puppyhood that these are not feathery chew toys. Don't skimp on this, because you're trusting this dog with your stock's lives, and you don't want to cheap-out on the price of a dog and pay for it with the lives of your stock.