Our dog (mutt, spaniel and whatever else is wandering the streets of Roanoke) has a huge prey drive. She once killed 4 squirrels in our fenced in 1/4 acre lot in the city… while she was still a puppy. She may have had a hand in harming a family member's cat. When we moved out here there was no fence, so at times that I could not be outside with her, we tied her out. Short periods, and this was until our we got our fence up. We did not have any other animals when she was a pup, so it was difficult to socialize her with anything other than dogs. I had one chicken in a small coop (long story) that I one day opened the door to let explore just a bit. Dog was tied outside at the same time and I ran back in the house for something. I peeked out the window at the dog only to see her gone… I stepped out and saw the poor chicken in a pile. My dog was a chicken killer. It just so happened that the day I opened up the coop door, my dog also slipped her collar. 100% not the dog's fault. My fault. I opened the door, I had the dog outside, and most importantly, I did not work with her as I should have around chickens.
I thought that was the end of it. She would be as she is forever. I was motivated by another person on BYC to use her to protect my chickens. Work with her. Teach her what is expected of her and what is not.
My husband and I took her out to a small enclosure with our 10 week old flock. We left her leash on her but let her loose with the chickens. We used our voice and the "leave it" command when she got too curious. We did not keep her from exploring and smelling the chickens, but we watched her body language closely and called her down at the first sign things were escalating. Tenseness, excitement were not acceptable. If calling down with our voice did not work we had the leash to apply pressure with our voices. We worked with her often, always ending on a good note until she all but ignored the chickens (though thoroughly enjoys the treats they leave behind

).
I can now say that today you would never know she killed a chicken a year ago. Heck, it's almost the opposite… we wish she would keep them away from the house!!! She will play with the rooster, but that is fair game ~ he sometimes starts the fights. She doesn't even look at the hens. I would not, at this point, trust her with chicks or anything still peeping… that is like a different animal all together and we have not worked on that yet.
I think all of this was possible because she respects us. We took the time to teach her what was acceptable instead of shaming her after the fact. We kept things very black and white and drew the line at the first sign she was acting inappropriately. I am so thankful for that person who convinced me that there was nothing wrong with her and that she needs to have a job and pull her own weight around the farm. I have no doubt that most would have returned this dog to the pound after having her for one week. For us, she has only gotten better with time. Just took giving her a chance to learn.
If you truly want to keep him/her, give him/her a chance to surprise you. Make the rules clear, but he/she must respect you first. Dogs want to please.

Good luck!!