English Mastiffs were bred to be war dogs. They were used by Roman soldiers in battle and to guard their camps, and eventually commoners used the dogs to guard their homes. This eventually led to bear baiting, lion hunting, and blood sport. Today, they are bred for docile temperament and companionship. I wouldn't necessarily use a Mastiff to guard a flock of birds.
I would personally go with a breed used specifically for guarding livestock or fowl. Australian Shepherds are so-named, not because they were bred in Australia (they are actually a Californian breed), but because they were bred to herd Australian geese specifically. This gives them a very gentle, calm nature with a hard-working attitude without the intensity and high-strung nature of a Border Collie (I had a Border Collie, too, so I feel experienced enough to speak on both
). Aussies are bred to work with birds by moving around them, where Border Collies work with cattle, usually with "The Eye" and by biting and nipping.
I got an Australian Shepherd two years ago with the intent of someday having chickens. He turned into an 80 lb teddy bear. He is very gentle and docile with a "soft mouth" (he doesn't bite down when things are put in his mouth). A few months ago, I got another Aussie. He's 6 months old and is an absolute terror in a way my first Aussie never was. Maybe it's that he's hitting puberty (he's starting to hump EVERYTHING), and he's running on straight testosterone. He gets the snip next Friday to calm him down, so I can start working on him with the chickens when we get them.
ANYWAY.... this is not an anti-Mastiff/Pit Bull/fighting dog post. Pit Bulls used to be bear-bait dogs, but they were then used to guard nurseries due to their loyal nature and desire to protect their owners and territory. Sure, you get a "cool factor" with the Mastiff, but are they the BEST choice for guarding livestock? Ehhh, I'm not so sure about that.
Any dog can be trained to do anything you want it to do. I feel the key is selecting a breed for your purpose, so that you don't have to "reprogram" a dog to do something he wasn't necessarily bred to do.
Plus, the cost of feeding a Mastiff is something to consider. Big dog, big food bowl, BIG POOPS!