That's awesome! Do you have a close up of his face?
Animalspooker, I would say that even though a Sheltie may not be able to fight off a predator, they would definitely still watch over their flock and property. And they will bark, which may be enough to deter an animal that was considering how easy of prey your flock would be. I guess it depends on what kinds of predators you have around. But as with any puppy, you have to teach them what their job is, and not leave them unsupervised until they are mature and trustworthy. It kind of sounds like the pups you had bad luck with from the pound might have been too young to be left on their own with the birds.
You might try a purebred rescue group if you want an economical dog. With any purebred, you will have a better idea of what the breed was developed for, as opposed to trusting your luck with an unknown mix. Also, the purebred rescue groups may know more about each dog's background and temperament, and be able to steer you towards a more suitable prospect. Good luck to you. I hope you find just the right dog, and have success in training it.